Pregnancy-associated thyroid disorders: the role of genetic, epigenetic, and oxidative stress factors

Abstract

Thyroid inflammation during pregnancy, particularly Hashimoto’s thyroiditis (HT) and postpartum thyroiditis (PPT), has a strong genetic and epigenetic basis. Susceptibility to these conditions is associated with specific HLA haplotypes (HLA-DR3, DR4, DR5) and immune-regulatory genes, including CTLA-4, PTPN22, FOXP3, as well as thyroid-specific genes such as TSHR, TG, and TPO. CTLA-4 polymorphism (CT60) is linked to increased thyroid autoantibody production, while PTPN22 R620W variant disrupts immune tolerance, exacerbating autoreactive lymphocyte activation.

Epigenetic modifications play a crucial role in HT and PPT pathogenesis. Dysregulation of microRNAs (miRNAs), including miR-146a, miR-142, miR-301, and miR-155, affects immune pathways by modulating T-cell responses and inflammatory cytokine production. Aberrant DNA methylation in genes regulating immune function, such as FOXP3 and CTLA-4, contributes to altered immune tolerance and disease progression.

Oxidative stress further modulates disease severity by inducing DNA damage and enhancing inflammatory responses, particularly in pregnancy. Reactive oxygen species (ROS) promote thyroid autoimmunity by affecting placental function and fetal neurodevelopment. Understanding the interplay between genetic susceptibility, epigenetic regulation, and oxidative stress is essential for developing personalized management strategies. This review highlights the molecular mechanisms underlying HT and PPT and the potential of epigenetic biomarkers for early diagnosis and targeted therapies.

Introduction

Thyroid inflammation related to pregnancy, particularly Hashimoto’s thyroiditis (HT) and postpartum thyroiditis (PPT), poses significant clinical challenges due to its impact on maternal metabolism, pregnancy complications, and fetal development [1, 2]. Women with a family history of thyroid disease or a history of infertility and recurrent miscarriages are particularly vulnerable and require close clinical monitoring [3]. HT and PPT conditions affecting approximately 5–15% of women of reproductive age, disrupt hormonal homeostasis, which is essential for maintaining a healthy pregnancy [4]. HT, the most common cause of hypothyroidism in pregnancy, affects about 2–3% of women and is characterized by chronic lymphocytic infiltration, progressive thyroid atrophy, and the presence of anti-thyroid antibodies, such as anti-thyroid peroxidase antibodies (TPOAb) and anti-thyroglobulin antibodies (TgAb) [5]. The clinical consequences of this condition are profound, as thyroid hormones play crucial role in early embryonic development and fetal neurogenesis [6, 7]. Even subclinical hypothyroidism or the mere presence of thyroid autoantibodies can increase the risk of pregnancy loss, implantation failure, and recurrent miscarriage [8]. Therefore, maternal thyroid dysfunction is associated with impaired placental function, fetal growth restriction, and long-term neurodevelopmental deficits in the newborn, including lower cognitive performance and an increased risk of attention deficits [9, 10]. Beyond fetal concerns, maternal health is also significantly affected by HT during pregnancy. Uncontrolled hypothyroidism is linked to a higher incidence of obstetric complications, including gestational hypertension, preeclampsia, preterm birth, placental abruption, and postpartum hemorrhage [2, 11]. Additionally, HT in pregnancy is increasingly recognized as a contributor to gestational diabetes mellitus and maternal metabolic dysregulation, which can have long-term consequences for both the mother and child [12, 13].

PPT, another major clinical concern, affects approximately 5–7% of postpartum women and typically follows a biphasic course, beginning with a transient thyrotoxic phase that transitions into hypothyroidism [14]. While the hyperthyroid phase may be mild and transient, the subsequent hypothyroid phase can be prolonged, leading to persistent fatigue, depression and mood disorders that are often misdiagnosed as postpartum depression [15]. Additionally, long-term maternal consequences include an increased risk of metabolic syndrome, cardiovascular disease, and insulin resistance, which underscores the need for adequate postpartum screening and early therapeutic intervention [16]. PPT condition also poses a risk to maternal-infant bonding, as untreated hypothyroidism may contribute to difficulties in lactation, reduced maternal immune responsiveness or adaptation during pregnancy, potentially influencing susceptibility to autoimmune thyroid conditions [17]. The changes in the nutritional composition of maternal milk related to PPT occurrence may significantly impact neonatal well-being, potentially leading to cognitive impairment or other morbidities [18].

A pathophysiological mechanism underlying pregnancy-associated thyroid disorders as HT and PPT involves a complex interplay of genetic predisposition, epigenetic modifications, immune dysregulation, and chronic inflammation, all of which contribute to oxidative stress and progressive thyroid dysfunction [19]. Increased oxidative damage during pregnancy may further impair placental function and contribute to fetal growth restriction, while in postpartum women, it may exacerbate thyroid dysfunction and accelerate the progression to permanent hypothyroidism [20, 21]. Genetic predisposition, notably through polymorphisms in key immune-regulatory genes and thyroid-specific genes, provides an initial susceptibility framework [22]. Epigenetic modifications, including aberrant DNA methylation and dysregulated microRNA (miRNA) expression, subsequently modulate this genetic vulnerability by influencing gene expression and immune responses [23]. These epigenetic mechanisms serve as pivotal intermediaries, bridging genetic predisposition and environmental influences such as nutritional status, stress, and hormonal fluctuations during pregnancy [23, 24]. Oxidative stress is intrinsically linked to genetic and epigenetic factors through pathways involving inflammation and immune modulation, as evidenced by the roles of miR-146a, miR-155, and DNA methylation of immune-related genes [25, 26]. Moreover, exosomes released under conditions of oxidative stress often contain altered molecular profiles that may contribute to the exacerbation of inflammation and the progression of HT and PPT [27]. They carry biologically active molecules such as proteins, mRNA, miRNA, and lipids, significantly contribute to cell-to-cell communication, impacting immune modulation and inflammatory responses. Proteomic analyses have identified altered exosomal protein profiles in patients with HT showing notable upregulation of proteins like hepatocyte growth factor-like protein (HGFL), focal adhesion kinase 1 (FAK1), and tyrosine-protein phosphatase non-receptor type 12 (PTPN12), and downregulation of proteins including proteasome inhibitor PI31 subunit (PSMF1) and cystatin-M (CYTM) playing key roles in regulating processes related to inflammation, immune response and proliferation [28].

Understanding this multifaceted relationship is critical for developing targeted diagnostic tools and therapeutic strategies aimed at mitigating oxidative stress and regulating immune responses, thus offering personalized treatment options for affected pregnancies [29, 30]. Given these wide-ranging implications, there is an urgent need for improved screening strategies, particularly for high-risk populations, to facilitate early diagnosis and optimize maternal and fetal outcomes [31]. Understanding the molecular mechanisms driving pregnancy-associated thyroid disorders could facilitate the development of targeted therapies, predictive biomarkers, and preventative strategies to optimize maternal and fetal outcomes [32].

This review aims to provide a comprehensive overview of pregnancy-associated thyroid disorders, with a particular focus on HT and PPT. It examines the interplay of genetic predisposition, epigenetic modifications, immune dysregulation, and oxidative stress in thyroid dysfunction, highlighting their impact on maternal metabolism, pregnancy outcomes, and fetal development. Furthermore, this review seeks to identify key pathophysiological mechanisms and potential biomarkers that could enhance early diagnosis, risk assessment, and personalized therapeutic strategies, ultimately improving maternal and neonatal health outcomes.

Diagnosis

The diagnosis of HT and PPT requires a comprehensive assessment of thyroid function, autoimmune markers, and imaging studies. The evaluation begins with measuring thyroid-stimulating hormone (TSH), free thyroxine (fT4), and free triiodothyronine (fT3) levels, which help determine the functional status of the thyroid gland [33]. In HT, TSH is typically elevated due to impaired thyroid hormone production, while fT4 and fT3 levels may be low or within the lower reference range, depending on disease progression. Subclinical hypothyroidism, a common early manifestation, is characterized by an elevated TSH level with normal fT4, whereas overt hypothyroidism presents with both increased TSH and decreased fT4 [34].

A crucial component of the diagnostic process is the detection of thyroid autoantibodies, primarily TPOAb and TgAb [35]. Elevated TPOAb and TgAb levels are hallmark indicators of autoimmune thyroiditis and may be detected years before the onset of clinical symptoms [36]. The presence of thyroid autoantibodies, particularly TPOAb, significantly increases the risk of developing PPT and progressing to permanent hypothyroidism, which can persist long after childbirth [1]. The complement cascade becomes activated under TPOAb synthesis, leading to elevated IgG1 levels, abnormalities in lymphocytes, increased natural killer (NK) cell activity, and associations with specific HLA haplotypes [37]. The inflammatory response triggers the breakdown of thyroglobulin within thyroid follicles, leading to follicular destruction and the release of large amounts of thyroxine (T4) and triiodothyronine (T3) into the bloodstream, resulting in a hyperthyroid state [38]. This hyperthyroid phase is temporary, persisting until the stored thyroglobulin is completely released into circulation and depleted from the follicles. During this period, the excessive levels of T4 and T3 suppress TSH secretion, leading to a shutdown of new hormone synthesis. Normal thyroid hormone production resumes once the inflammatory process resolves [14].

In addition to biochemical markers, thyroid ultrasonography plays a vital role in diagnosing and assessing disease severity. Typical ultrasonographic findings in HT include a heterogeneous, hypoechoic thyroid texture with increased vascularization, which reflects the chronic inflammatory process and lymphocytic infiltration [39]. The presence of hyperechoic or fibrotic changes is associated with more advanced disease stages and a higher likelihood of developing permanent hypothyroidism. In pregnant women, hypoechoic patterns combined with elevated TPOAb levels serve as predictive indicators for the subsequent onset of PPT [40].

The diagnosis of PPT follows a distinct pattern, characterized by an initial thyrotoxic phase, followed by transient hypothyroidism. In the early phase, laboratory findings reveal suppressed TSH levels with elevated fT4 and fT3, mimicking gestational thyrotoxicosis or Graves’ disease [41]. Differentiation from Graves’ disease is critical, as the latter is associated with positive thyroid-stimulating immunoglobulin (TSI) and persistent hyperthyroidism [42]. In contrast, PPT is self-limiting, with thyrotoxicosis typically resolving within a few weeks [43]. The second phase of PPT is marked by hypothyroidism, with an increase in TSH and a reduction in fT4 levels. This phase can last several months, and in approximately 20–50% of affected women, it progresses to permanent hypothyroidism [14].

An important emerging diagnostic tool is the fT4-to-TSH ratio, which has been suggested as a predictor of thyroid dysfunction during pregnancy and postpartum [44]. A lower fT4-to-TSH ratio in early pregnancy has been linked to an increased risk of developing hypothyroidism later in gestation or after delivery [45]. Additionally, ultrasonographic findings such as reduced thyroid volume and diffuse hyperechogenicity may indicate an increased susceptibility to long-term thyroid dysfunction [46].

Given the significant clinical implications of thyroid inflammation during and after pregnancy, early identification of high-risk individuals is crucial [47]. Screening for TSH and TPOAb in the first trimester, particularly in women with a history of infertility, recurrent miscarriage, or thyroid disease, allows for timely intervention and improved pregnancy outcomes. The presence of thyroid autoimmunity, even in euthyroid individuals, warrants close postpartum monitoring, as these women have a significantly increased risk of developing PPT and subsequent permanent hypothyroidism [48, 49].

Accurate and timely diagnosis is essential for optimizing maternal and fetal health, guiding appropriate treatment strategies, and reducing the risk of long-term complications associated with thyroid dysfunction [50] (Table 1).

Table 1.

HT and PPT characterization

Category	Hashimoto’s Thyroiditis	Postpartum Thyroiditis
Prevalence	~ 2–3% of pregnant women	~ 5–7% of postpartum women
Genetic Factors	HLA-DR3, DR4, DR5, CTLA-4, PTPN22, FOXP3, TSHR, TG	HLA-DR3, DR4, DR5, CTLA-4 CT60 polymorphism
Epigenetic Regulation	Aberrant DNA methylation in FOXP3, CTLA-4; miR-146a, miR-142, miR-301, miR-155 dysregulation	Limited data; potential involvement of DNA methylation and miRNA pathways
Pathophysiology	Chronic autoimmune lymphocytic infiltration, progressive thyroid atrophy, elevated TPOAb, TgAb	Biphasic: transient thyrotoxicosis → hypothyroidism; TPOAb-associated inflammation
Oxidative Stress Role	Increased ROS, lipid peroxidation, mitochondrial dysfunction contributing to thyroid destruction	ROS overproduction postpartum, leading to follicular apoptosis and inflammation
Clinical Implications	Hypothyroidism, miscarriage, preterm birth, preeclampsia, impaired fetal neurodevelopment	Fatigue, depression, increased risk of metabolic syndrome, impaired lactation
Diagnostic Markers	↑TSH, ↓fT4, positive TPOAb, TgAb; hypoechoic thyroid on ultrasound	Initial: ↓TSH, ↑fT4 (hyperthyroid phase); Later: ↑TSH, ↓fT4 (hypothyroid phase)
Therapeutic Approaches	Levothyroxine, antioxidant supplementation (selenium, vitamins C, E, omega-3)	Monitoring, symptomatic management, potential need for levothyroxine in prolonged hypothyroidism
Future Research Directions	DNA methylation as biomarker, miRNA-targeted therapies, immune modulation	Epigenetic biomarkers for prediction, antioxidant therapy to reduce disease severity

Genetic background

Notably, autoimmune thyroid diseases, including HT and PPT, have been found to occur more frequently in individuals with specific genetic syndromes, such as Turner syndrome, Down syndrome, and Klinefelter syndrome. This suggests that underlying chromosomal abnormalities may contribute to thyroid autoimmunity in susceptible populations [51]. Nevertheless, genetic research has provided substantial evidence supporting the role of hereditary factors in the development of HT and PPT.

1. HT

HT exhibit a strong genetic predisposition, as demonstrated by familial aggregation, twin studies, and genome-wide association studies (GWAS) [52]. The study conducted in Croatia on 1,443 individuals aimed to identify genetic determinants of HT through the first GWAS dedicated exclusively to this disease. In the discovery phase, an association analysis was performed on 405 cases and 433 controls, followed by replication in two independent cohorts comprising 303 cases and 302 controls. Meta-analysis of the discovery and replication datasets identified three genetic variants suggestively associated with HT: rs12944194 located near SDK2 (P = 1.8 × 10⁻⁶), rs75201096 within GNA14 (P = 2.41 × 10⁻⁵), and rs791903 inside IP6K3 (P = 3.16 × 10⁻⁵). A genetic risk score (GRS) based on these variants explained 4.82% of the total variance in HT risk, with individuals in the highest GRS quartile having 2.76 times higher odds of developing HT than those in the lowest quartile. Although these loci were associated with HT for the first time, they are located in biologically relevant regions involved in thyroid function and autoimmunity. Furthermore, a genetic overlap between HT and related conditions, such as hypothyroidism, Graves’ disease, and TPOAb levels, was observed. These findings provide novel insights into HT genetics and establish a foundation for future research [52]. The following study conducted by Wei et al. on 85,421 pregnant women in China represents the largest GWAS investigating thyroid-related traits, including thyroid hormones, dysfunction, and autoimmunity during pregnancy. The research identified 176 genetic loci, of which 125 were novel, providing new insights into the genetic basis of thyroid function in pregnancy. The study focused on eight thyroid traits, including TSH, FT4, and TPOAb levels, as well as four forms of thyroid dysfunction: subclinical hypothyroidism, isolated hypothyroxinemia, overt hyperthyroidism, and subclinical hyperthyroidism. Through a meta-analysis of GWAS data from two hospitals, the study identified 38 loci associated with TSH levels and 22 loci linked to FT4 levels, with several novel genetic associations, including heterogeneous nuclear ribonucleoprotein A3 (HNRNPA3), fibroblast growth factor 10 (FGF10), insulin-like growth factor 1 (IGF1) and melanoma associated antigen D2 (MAGED2), thyrotropin-releasing hormone (TRH) relevant for TSH synthesis and TSH receptor (TSHR) functioning, phosducin-like protein 2 (PDCL2), glycoprotein hormones, alpha polypeptide (CGA), and zinc finger protein 462 (ZNF462) for FT4. Variations in these loci may increase susceptibility to pregnancy-associated thyroid dysfunction, impacting maternal and fetal health. HNRNPA3 influences TSH regulation through RNA processing, while FGF10 and IGF1, part of the FGF and IGF signaling pathways, are essential for thyroid development and hormone balance. MAGED2 plays a role in cellular stress responses, impacting thyroid hormone regulation, particularly during pregnancy. The HPT axis is directly affected by TRH, which regulates TSH secretion, while CGA is essential for TSH biosynthesis. Additionally, PDCL2 contributes to G-protein-coupled receptor (GPCR) signaling, affecting TSHR activation, and ZNF462, involved in epigenetic regulation, may modulate thyroid-specific gene expression. GPCR signaling is essential for thyroid hormone regulation, given that the TSH receptor is a member of the GPCR superfamily. When TSH binds to TSHR, it activates Gαs proteins, stimulating adenylyl cyclase (AC) and increasing cAMP levels. This triggers protein kinase A, which upregulates thyroid hormone synthesis enzymes (thyroid peroxidase, sodium-iodide symporter), promoting T4 and T3 production. Understanding these mechanisms could help improve early diagnosis and targeted therapies. Importantly, the study established genetic correlations between thyroid function in pregnancy and long-term health outcomes, such as gestational complications, birth outcomes, and later-life thyroid and cardiovascular diseases. Mendelian randomization (MR) analysis suggested that genetically higher TSH levels were associated with lower glucose levels, reduced blood pressure, and longer gestational duration, while TPOAb positivity was linked to increased risks of Graves’ disease, Hashimoto’s thyroiditis, and cardiac valvular disease. These findings enhance the understanding of the genetic determinants of thyroid function during pregnancy and suggest potential new approaches for early diagnosis, preventive strategies, and therapeutic interventions [53].

One of the most significant genetic factors associated with HT is the human leukocyte antigen - DR gene (HLA-DR), particularly the HLA-DR3, DR4, and DR5 alleles, which influence antigen presentation and immune response regulation [54, 55]. Studies have demonstrated that the presence of arginine at position 74 of the DRβ1 chain is a strong predictor of susceptibility to autoimmune thyroid disease [55, 56]. Another well-established genetic factor is cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4), a key immune checkpoint molecule regulating T-cell activation. A polymorphism at position 49 in the CTLA-4 has been linked to increased susceptibility to both HT and Graves’ disease, reinforcing its role in autoimmunity [57]. Similarly, protein tyrosine phosphatase non-receptor type 22 (PTPN22), a gene involved in T-cell receptor signaling, has been found to harbor a R620W polymorphism, which significantly increases the risk of HT by altering immune tolerance [58]. Furthermore, FOXP3, a gene essential for the development of regulatory T cells, has been implicated in HT, with promoter-region polymorphisms affecting Treg function and contributing to thyroid autoimmunity [59]. Furthermore, genetic predisposition accounts for approximately 80% of the risk of developing HT [60, 61]. The TSHR located on chromosome 14q31, a key regulator of thyroid function, has been studied in autoimmune thyroid diseases. While strongly linked to Graves’ disease, certain TSH-R polymorphisms may also influence HT by altering receptor sensitivity, contributing to thyroid cell destruction. The rs179247-A allele has been associated with higher susceptibility to HT, potentially by modifying TSH receptor availability on thyroid cells, when rs2268458 (G/A variant) affects TSHR promoter activity, leading to altered receptor expression levels [62]. Identifying TSHR polymorphisms could help stratify the risk of developing HT, especially in individuals with a family history of thyroid disease. These variants may also predict disease progression, particularly the likelihood of permanent hypothyroidism in HT patients. Understanding TSHR genetic variations could improve individualized treatment strategies, especially in tailoring levothyroxine therapy or assessing risk for more aggressive disease courses.

Among other thyroid-specific genes, thyroglobulin (TG) gene polymorphisms (chromosome 8q24) have been significantly associated with HT susceptibility, affecting immune response and thyroid inflammation [63]. Studies on the thyroid peroxidase gene (TPO) (chromosome 2p25) and thyroid hormone receptor β gene (THRB) (chromosome 3p24) have shown conflicting results, requiring further research [64–66]. Specifically, while several studies have identified TPO gene polymorphisms (such as rs732609 and rs11675434) as potential risk factors for HT, others have not confirmed these associations, possibly due to population-specific genetic backgrounds or methodological differences. Similarly, although some data suggest that THRB gene variants may affect thyroid hormone action and contribute to immune dysregulation, these findings have not been consistently replicated, highlighting the need for further well-powered studies in diverse cohorts (Table 2).

Table 2.

Genetic risk factors for HT

Genetic Factor / Locus	Description / Location	Significance for HT
HLA-DR3, DR4, DR5	Tissue compatibility system class II genes, crucial for antigen presentation	Effects on autoantigen presentation and development of autoimmune response
CTLA-4 (+ 49 polymorphism)	Regulator of T-cell activation; immune checkpoint	Polymorphism linked to HT and Graves-Basedow disease; impaired regulation of T cells
PTPN22 (R620W)	Tyrosine phosphatase involved in T-lymphocyte signaling	Increased risk of HT through impaired immune tolerance
FOXP3	A transcriptional regulator of Treg lymphocytes	Promoter region polymorphisms impair Treg function, increasing the risk of autoimmunity
TSH-R	TSH receptor, chromosome 14q31	Mainly associated with Graves-Basedow disease, but certain polymorphisms can affect HT; change in receptor sensitivity; which contributes to thyroid cell destruction
TG	Thyroglobulin, chromosome 8q24	Polymorphisms affect Tg immunogenicity, altering immune response increasing HT risk
TPO	Thyroid peroxidase, chromosome 2p25	Variable correlation with HT, affects TPOAb antibody production; needs further study.
THRB	Thyroid hormone β receptor, chromosome 3p24.	Results inconclusive; role in HT not fully established
HNRNPA3, FGF10, IGF1, MAGED2, TRH, PDCL2, CGA, ZNF462	-	Potential new diagnostic and therapeutic targets related to thyroid hormones, TSH and FT4 levels, and autoimmunity
	SNP
rs75201096 (GNA14)	-	Variant within GNA14; potentially associated with HT (P = 2.41 × 10-⁵).
rs791903 (IP6K3)	-	Variant in IP6K3; associated with HT risk (P = 3.16 × 10-⁵).
rs12944194 (SDK2)	-	Localized near SDK2; suggested role in HT (P = 1.8 × 10-⁶).

HLA – Human Leukocyte Antigen; CTLA-4 – Cytotoxic T-Lymphocyte Antigen 4; PTPN22 – Protein Tyrosine Phosphatase Non-Receptor Type 22; FOXP3 – Forkhead Box P3; TSH-R – Thyroid-Stimulating Hormone Receptor; TG – Thyroglobulin; TPO – Thyroid Peroxidase; THRB – Thyroid Hormone Receptor Beta; HNRNPA3 – Heterogeneous Nuclear Ribonucleoprotein A3; FGF10 – Fibroblast Growth Factor 10; IGF1 – Insulin-Like Growth Factor 1; MAGED2 – Melanoma-Associated Antigen D2; TRH – Thyrotropin-Releasing Hormone; PDCL2 – Phosducin-Like 2; CGA – Glycoprotein Hormones, Alpha Polypeptide; ZNF462 – Zinc Finger Protein 462; SNP – Single Nucleotide Polymorphism; HT – Hashimoto’s Thyroiditis; PPT – Postpartum Thyroiditis; TPOAb – Thyroid Peroxidase Antibodies; Treg – Regulatory T cell; ROS – Reactive Oxygen Species; 8-OHdG – 8-Hydroxy-2’-deoxyguanosine; 8-oxo-dG – 8-oxo-7,8-dihydro-2′-deoxyguanosine

• b. PPT

In PPT, genetic susceptibility has been strongly associated with HLA-DR3, DR4, and DR5 haplotypes, which are linked to an increased immune response against thyroid autoantigens [67, 68]. The CT60 polymorphism in the CTLA-4 gene has been identified as a significant factor influencing thyroid autoantibody production in patients with HT and PPT. Research indicates that individuals carrying the G allele of this polymorphism exhibit higher levels of TPOAb and TgAb, suggesting a genetic predisposition to heightened autoimmune activity in these thyroid disorders. Furthermore, in PPT patients, the CT60 polymorphism appears to affect thyroid function, with G allele carriers being more prone to developing the hypothyroid form of the disease. These findings underscore the role of CTLA-4 gene variations in modulating immune tolerance and inflammatory responses in thyroid autoimmunity [69] (Table 3).

Table 3.

Genetic risk factors for PPT

Genetic factor	Description / Gen	Relevance to PPT
HLA-DR3, DR4, DR5	Haplotypes of the HLA class II system	Increased immune response to thyroid autoantigens, strong genetic predisposition to PPT
CTLA-4 (CT60 polymorphism, G allele)	T-4 cytotoxic lymphocyte antigen gene	Increased production of TPOAb and TgAb, increased risk of postpartum hypothyroidism, impaired immune tolerance

HLA – Human Leukocyte Antigen; DR – DR locus (within HLA class II); CTLA-4 – Cytotoxic T-Lymphocyte Antigen 4; CT60 – Cytotoxic T-Lymphocyte Antigen 4, position 60 polymorphism; TPOAb – Thyroid Peroxidase Antibodies; TgAb – Thyroglobulin Antibodies; PPT – Postpartum Thyroiditis

Epigenetic regulation in HT and PPT

Recent epigenetic studies suggest that miRNAs play a crucial role in the regulation of gene expression associated with HT and PPT. A systematic review identified three key miRNAs—miR-146a, miR-142, and miR-301—that are dysregulated in HT, contributing to immune response modulation and inflammation. Furthermore, oxidative stress-induced miRNA dysregulation may amplify autoimmune pathways. For example, miR-146a upregulation in response to ROS enhances NF-κB activity, creating a positive feedback loop that perpetuates inflammation. Additionally, variants in CTLA-4 and PTPN22 genes alter T-cell activation thresholds, reducing immune tolerance and promoting autoreactivity. DNA methylation defects in FOXP3 impair Treg cell development, leading to uncontrolled inflammatory responses. Collectively, these molecular mechanisms underscore how genetic susceptibility, epigenetic misregulation, and oxidative stress converge to drive thyroid autoimmunity during pregnancy [70–72]. MiR-142 affects the janus kinase-signal transducer and activator of transcription pathway (JAK-STAT), impairing regulatory T-cell (Treg) function and enhancing autoreactive T-cell activation, contributing to immune dysregulation in HT. Meanwhile, miR-301 influences the phosphoinositide 3-kinase/AKT/mechanistic target of rapamycin pathway (PI3K-AKT-mTOR) pathway, leading to prolonged T-cell survival and increased autoantibody production, exacerbating autoimmune attack on thyroid tissue [73]. Additionally, miR-21-5p has been found to be significantly upregulated in HT patients, whereas miR-150-5p is involved in the differentiation and activation of immune cells, suggesting a potential role in adaptive immunity [74]. These findings indicate that miRNA dysregulation may modify antigen responses, further influencing the pathogenesis of autoimmune thyroid diseases [75].

Beyond miRNA involvement, DNA methylation plays a fundamental role in gene expression regulation and cellular homeostasis. Aberrant methylation patterns have been linked to improper gene activation or silencing, which may contribute to the development of HT and PPT [76, 77]. The study conducted by Wenqian et al. on 13 pregnant women with HT and 8 healthy pregnant women as a control group revealed significant differences in DNA methylation patterns between the two groups. A total of 652 differentially methylated positions (DMPs) and 27 differentially methylated regions (DMRs) were identified. Gene Ontology (GO) analysis indicated that DMPs were significantly enriched in 540 GO terms, including regulation of keratinocyte differentiation, T helper cell differentiation, and alpha-beta T-cell differentiation. Additionally, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed significant associations with mucin-type O-glycan biosynthesis, focal adhesion, and the insulin signaling pathway. GO terms related to DMRs included muscle cell proliferation, response to biotic stimulus, and anatomical structure formation involved in morphogenesis, with genes primarily implicated in the FoxO signaling pathway. Furthermore, the DTNA gene was identified as the seed gene of functional epigenetic modules. These findings indicate that the DNA methylation pattern in the HT group differs significantly from that of the control group, suggesting that epigenetic modifications may contribute to the development of HT by regulating autoimmune processes [71].

DNA methylation in PPT has not been extensively studied, making it a subject for further investigation. Moreover, prenatal folic acid supplementation, particularly before conception and during the first trimester of pregnancy, has been shown to enhance DNA methylation, potentially influencing fetal development and reducing the risk of neural tube defects [78]. Folic acid, as a crucial methyl donor in one-carbon metabolism, directly contributes to the availability of S-adenosylmethionine (SAM), the universal methyl group donor for DNA methylation reactions [79, 80]. Aberrant methylation patterns in regulatory regions of immune-related genes, such as FOXP3 and IL-10, may impair immune homeostasis, potentially increasing susceptibility to autoimmune responses targeting the thyroid gland [81]. Consequently, variations in folate intake before and during pregnancy could modulate the epigenetic landscape, thus affecting the risk and progression of autoimmune thyroid diseases, including PPT. These findings emphasize the importance of epigenetic modifications in thyroid autoimmunity and suggest that further research may lead to novel diagnostic and nutritional therapeutic approaches [82] (Table 4).

Table 4.

Key genes and epigenetic markers in HT and PPT

Gene/Epigenetic Marker	Function	Relation to Thyroid Function	Link to Oxidative Stress	Clinical Outcomes
HLA-DR3/DR4/DR5	Antigen presentation	Associated with autoantigen recognition	Indirect via immune activation	HT/PPT susceptibility, miscarriage risk
CTLA-4 (CT60)	Inhibits T-cell activation	Reduces immune tolerance	Exacerbated inflammation	HT, PPT, increased TPOAb
PTPN22 (R620W)	Modulates T-cell receptor signaling	Promotes autoreactive T-cell survival	ROS-related immune activation	HT, risk of permanent hypothyroidism
FOXP3	Treg cell differentiation	Loss of immune suppression	Inflammatory stress sensitivity	HT, impaired fetal tolerance
TSHR	TSH receptor function	Alters thyroid hormone regulation	Promotes ROS when dysregulated	Gestational thyroid dysfunction
TG	Thyroglobulin synthesis	Target of autoantibodies	Subject to oxidative attack	HT, fetal neurodevelopment issues
miR-146a	Regulates NF-κB and cytokines	Promotes inflammation in thyroid tissue	Upregulated by ROS	HT severity, cytokine dysregulation
miR-155	Modulates inflammatory responses	Enhances autoimmunity	ROS enhances expression	Autoimmune thyroid flare
DNA methylation (FOXP3)	Epigenetic silencing of immune genes	Reduces Treg function	Affected by redox balance	HT in pregnancy, preeclampsia
8-OHdG	Oxidative DNA damage marker	Indicates oxidative damage in thyroid	Direct marker of oxidative stress	Placental dysfunction, cognitive deficits

HLA – Human Leukocyte Antigen; DR – DR locus (class II MHC); CTLA-4 – Cytotoxic T-Lymphocyte Antigen 4; TPOAb – Thyroid Peroxidase Antibodies; PTPN22 – Protein Tyrosine Phosphatase Non-Receptor Type 22; ROS – Reactive Oxygen Species; FOXP3 – Forkhead Box P3; Treg – Regulatory T cell; TSHR – Thyroid-Stimulating Hormone Receptor; TG – Thyroglobulin; miR-146a – microRNA-146a; miR-155 – microRNA-155; NF-κB – Nuclear Factor kappa-light-chain-enhancer of activated B cells; DNA methylation – DNA-based epigenetic gene silencing; 8-OHdG – 8-Hydroxy-2′-deoxyguanosine; HT – Hashimoto’s Thyroiditis; PPT – Postpartum Thyroiditis

Oxidative stress

Genetic and epigenetic studies have demonstrated that susceptibility to autoimmune thyroid disorders, such as HT and PPT, is influenced by specific genetic variants as well as regulatory mechanisms, including DNA methylation and miRNA expression. However, beyond hereditary factors, oxidative stress plays a crucial role in exacerbating immune dysfunction and thyroid cell damage [83, 84].,, particularly during pregnancy and the postpartum period [85]. Oxidative stress results from an imbalance between the production of ROS and the body’s ability to neutralize them through antioxidant defenses [86]. The thyroid gland is particularly susceptible to oxidative damage due to its high metabolic activity and reliance on hydrogen peroxide for thyroid hormone synthesis [87].

1. Oxidative Stress in HT During Pregnancy

Studies have demonstrated that in pregnancy, immune modulation is crucial for maintaining fetal tolerance; however, in women with HT, persistent autoimmune activation exacerbates oxidative stress as presented in Fig. 1 (Fig. 1). Research indicates that HT patients exhibit elevated levels of malondialdehyde (MDA), a biomarker of lipid peroxidation, along with reduced activity of antioxidant enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GPx). This oxidative imbalance contributes to chronic thyroid inflammation and may further deteriorate thyroid function during pregnancy, increasing the risk of hypothyroidism, preeclampsia, gestational diabetes, and adverse fetal outcomes [88, 89].

Fig. 1.

Oxidative Stress Pathomechanisms in HT and PPT

CTLA-4 – Cytotoxic T-Lymphocyte Antigen 4; PTPN22 – Protein Tyrosine Phosphatase Non-Receptor Type 22; FOXP3 – Forkhead Box P3; TSHR – Thyroid-Stimulating Hormone Receptor; TG – Thyroglobulin; TPO – Thyroid Peroxidase; miR-146a – microRNA-146a; miR-155 – microRNA-155; HT – Hashimoto’s Thyroiditis; PPT – Postpartum Thyroiditis

Furthermore, TPOAb-positive women, even in euthyroid states, have been found to exhibit increased oxidative stress markers, suggesting that oxidative damage may precede overt hypothyroidism and impair pregnancy adaptation mechanisms [90, 91]. Elevated oxidative stress markers such as 8-oxo-7,8-dihydroguanosine (8-oxoGuo) and 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxodG) have been identified in pregnant women with thyroid dysfunction, reinforcing the association between oxidative damage and thyroid autoimmunity [92].

Additionally, studies indicate that the presence of TPOAb during pregnancy correlates with detrimental morphological changes in the placenta, increased expression of pro-inflammatory cytokines, and elevated oxidative stress markers, all of which may impact fetal development and pregnancy outcomes [93]. These findings underscore the necessity of monitoring oxidative stress in pregnant women with HT and TPOAb positivity to mitigate potential complications.

• b. Oxidative Stress in PPT – future perspectives studies

PPT, an autoimmune-mediated transient thyroid dysfunction, also involves increased oxidative stress, particularly in women with preexisting thyroid autoimmunity. The postpartum period is characterized by a rapid shift in immune system activity, as immune suppression during pregnancy is reversed to restore normal function. This immune rebound, coupled with ROS overproduction, contributes to thyroid cell apoptosis and dysfunction in susceptible individuals [94].

In the thyrotoxic phase of PPT, inflammatory response along with an excessive ROS production, leads to thyroid follicular cell destruction, releasing stored thyroid hormones. Markers of oxidative stress, including higher levels of nitric oxide (NO) and lipid peroxidation products, have been found in postpartum women with PPT [83, 95]. Subsequently, in the hypothyroid phase, persistent oxidative stress may contribute to mitochondrial dysfunction in thyroid cells, leading to an increased risk of permanent hypothyroidism [96].

Moreover, oxidative stress in PPT has been linked to postpartum depression and fatigue, conditions commonly observed in women with PPT. Increased levels of oxidative markers, including 8-hydroxy-2′-deoxyguanosine (8-OHdG), a marker of DNA damage, suggest that prolonged oxidative stress may contribute to mood disturbances and overall maternal well-being. Most importantly, recent studies have emphasized the critical role of oxidative stress in the pathophysiology of PPT. Riis et al. demonstrated that postpartum women with hypothyroidism exhibit persistently elevated levels of 8-OHdG, a marker of oxidative DNA damage, suggesting sustained oxidative stress beyond the immediate postpartum period [97]. Similarly, Halczuk et al. reported increased concentrations of 8-oxo-dG and MDA in TPOAb-positive pregnant women, indicating that oxidative damage may precede the onset of clinical hypothyroidism. Inflammatory responses triggered by ROS, including elevated nitric oxide (NO) levels, were also observed in experimental and clinical settings, further linking oxidative stress to follicular cell apoptosis and dysregulation of thyroid hormone synthesis [98, 99]. These findings support the potential utility of oxidative stress biomarkers for early diagnosis and prognosis in PPT and highlight possible therapeutic targets for antioxidant-based interventions [100].

• c. Clinical Implications and Therapeutic Perspectives

Given the critical role of oxidative stress in both HT during pregnancy and PPT, antioxidant-based therapies have been proposed as potential adjunctive treatments. Studies suggest that selenium supplementation, known for its antioxidant properties, may reduce thyroid autoantibody levels and improve thyroid function in women with autoimmune thyroid disease [101]. Additionally, dietary antioxidants such as vitamins C and E, polyphenols, and omega-3 fatty acids may help mitigate ROS-induced damage and improve pregnancy outcomes in women with HT by modulating key redox-sensitive and inflammatory signaling pathways [102, 103]. Vitamin C (ascorbic acid) functions as a potent ROS scavenger by donating electrons to neutralize free radicals. It enhances the activity of the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway, which upregulates antioxidant response elements (AREs) such as SOD, catalase, and GPx, reducing oxidative stress in thyroid tissue and preventing lipid peroxidation [104]. Vitamin E (tocopherol) stabilizes cell membranes by inhibiting lipid peroxidation and interacts with the NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) pathway, reducing the expression of pro-inflammatory cytokines like TNF-α, IL-6, and IL-1β, thereby mitigating thyroid inflammation [105]. Polyphenols, such as flavonoids, exert antioxidant and anti-inflammatory effects by modulating AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1) signaling, enhancing mitochondrial function and reducing oxidative damage in immune cells [106]. Omega-3 fatty acids (DHA and EPA) regulate inflammation through the PPAR-γ (peroxisome proliferator-activated receptor gamma) and GPR120 (G-protein coupled receptor 120) pathways, inhibiting pro-inflammatory eicosanoids derived from arachidonic acid and promoting the production of specialized pro-resolving mediators (SPMs), such as resolvins and protectins [107, 108]. Through these mechanisms, these dietary antioxidants help maintain redox balance, modulate immune responses, and improve endothelial and placental function, ultimately supporting better pregnancy outcomes in women with HT. These dietary antioxidants help maintain redox balance, modulate immune responses, and improve endothelial and placental function through distinct molecular mechanisms, including Nrf2 activation (vitamin C) [109], NF-κB inhibition (vitamin E) [105], AMPK/SIRT1 modulation (polyphenols) [110] and pro-resolving mediator synthesis via GPR120 (omega-3 fatty acids) [108].

In PPT, monitoring oxidative stress markers could serve as an early predictor of disease progression, allowing for timely interventions to prevent permanent hypothyroidism [111]. Antioxidant strategies, combined with appropriate thyroid hormone replacement therapy, when necessary, may contribute to better maternal recovery and reduced long-term complications [112].

Overall, oxidative stress is a key driver of thyroid dysfunction in pregnancy and PPT, influencing disease progression and clinical outcomes [87]. Further research into oxidative stress biomarkers and targeted antioxidant therapies may offer new strategies for managing thyroid autoimmunity in pregnant and postpartum women.

Conclusions

Thyroid inflammation in pregnancy, particularly HT and PPT, results from a complex interplay of genetic, epigenetic, and environmental factors. Genetic predisposition, including HLA-DR3, DR4, DR5, CTLA-4, PTPN22, and FOXP3 polymorphisms, plays a crucial role in immune dysregulation and thyroid autoimmunity. These genetic markers may help predict disease susceptibility and progression, particularly the risk of permanent hypothyroidism. Epigenetic mechanisms, particularly miRNA dysregulation and DNA methylation, significantly contribute to disease development and progression. Altered expression of miR-146a, miR-155, miR-301, and miR-223 disrupts immune tolerance, while aberrant DNA methylation in immune-modulatory genes (FOXP3, CTLA-4) exacerbates thyroid dysfunction. These findings underscore the potential of epigenetic biomarkers for early detection, risk assessment, and targeted therapeutic interventions. Oxidative stress amplifies thyroid damage, particularly in the postpartum period, by increasing ROS production, mitochondrial dysfunction, and inflammatory cytokine activation. Addressing oxidative stress through antioxidant therapy (selenium, vitamins C and E, omega-3 fatty acids) may help mitigate disease progression. Given the broad clinical implications of thyroid inflammation in pregnancy, early screening and personalized treatment approaches that incorporate genetic, epigenetic, and oxidative stress markers should be prioritized. Future research should focus on the development of DNA methylation-modifying agents and miRNA-targeted therapies, which could offer novel preventive and therapeutic strategies for autoimmune thyroid diseases during pregnancy and postpartum.

Author contributions

Conceptualization, A.B.; methodology, A.B. and J.H; software, A.B. and I.S.; validation, A.B. and I.S.; formal analysis, A.B. and M.ZK.; investigation, A.B. and J.Dz.; resources, AB.; data curation, A.B.; writing—original draft preparation, A.B.; writing—review and editing, I.S. and A.J.K; visualization, A.B.; supervision, A.B., M.Sz and A.J.K.; project administration, A.B.; funding acquisition, A.B. All authors have read and agreed to the published version of the manuscript.

Funding

The study was supported by the funds of the internal financing of Medical University of Bialystok (B.SUB.24.547).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.

Institutional review board

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Informed consent

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Conflict of interest

The authors declare no conflicts of interest.

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Clinical trial number

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