Case Report

Abstract

Background

Thyroid hormone resistance (RTH) is defined as a decrease in response to thyroid hormones in the target tissue. Most patients present with nonspecific findings. In this article, we aimed to represent a 22-year-old female patient who presented with palpitation, fatigue, and heat intolerance. She was thought to have thyroid hormone resistance and her genetic examination revealed NM_001128177.1 (THRβ): c.1034G > A (p.Gly345Asp) pathogenic variation in the THRβ gene.

Case report

A 22-year-old female patient presented with complaints of fatigue, heat intolerance and palpitations. She was taking Propranolol twice daily at admission. Her family history revealed hypothyroidism in her grandmother. Her physical examination results were as follows: height 160 cm, weight 65 kg, body mass index 25.4kg/m², body temperature 36.5°C, respiratory rate 18/min, heart rate 86 beats/min, blood pressure 120/80 mmHg. Her palms were sweaty. The heart sounds were normal, and no heart murmur was auscultated. The laboratory results were TSH: 5.31uU/mL, fT3: 6.83 pg/mL, and fT4: 2.43 ng/dL. THRβ gene mutation analysis was requested for our patient whose clinical history and laboratory results were compatible with thyroid hormone resistance. The pathogenic variation NM_001128177.1(THRβ):c.1034G>A (p.Gly345Asp) was detected after analysis.

Conclusion

A diagnosis of RTH requires high clinical suspicion and a genetic mutation analysis should be requested in the case of clinical suspicion. In this way, unnecessary anti-thyroid treatment can be prevented.

Introduction

Thyroid hormone resistance (RTH) is defined as a decrease in response to thyroid hormones in the target tissue (1). RTH, also known as Refetoff syndrome, was first described in 1967 by Refetoff. Although free thyroxine (fT4) and triiodothyronine (fT3) levels increase, there is a level of unsuppressed thyroid-stimulating hormone (TSH) (2). With the incidence of 1:40,000, it is a genetic disorder with autosomal dominant or autosomal recessive inheritance. Approximately 15% of Refetoff syndrome cases are sporadic (1,2). RTH is most commonly caused by mutations in the Thyroid Hormone Receptor Beta (THRβ) gene located on chromosome 3. Most patients present with nonspecific findings (3). In this article, we aimed to represent a 22-year-old woman who presented with complaints of palpitation, fatigue, heat intolerance and thyroid hormone resistance. She was thought to have thyroid hormone resistance and her genetic examination revealed NM_001128177.1(THRβ): c.1034G > A (p.Gly345Asp) pathogenic variation in the THRβ gene.

Case Presentation

A 22-year-old female patient with complaints of fatigue, heat intolerance and palpitations was admitted to our clinic. Her medical history revealed that two years ago when she did a tympanoplasty, her laboratory results were TSH: 5.31mU/L, fT3: 6.83 pg/mL, and fT4: 2.43 ng/dL. At the time of her admission; she had been taking Propranolol twice daily for two years and methimazole 10 mg daily for one year. We learned that the patient’s complaints did not reduce although her treatment was uninterrupted. Her family history revealed hypothyroidism in her grandmother. Her physical examination results were as follows: height 160 cm, weight 65 kg, body mass index 25.4 kg/m², body temperature 36.5°C, respiratory rate 18/min, heart rate 86 beats/min, blood pressure 120/80 mmHg. Her palms were sweaty. Her heart sounds were normal, and no heart murmur was auscultated. Pretibial edema, tremors, or ophthalmopathy were not observed. Thyroid examination revealed no nodule or palpable hard mass. Laboratory results are given in Table 1. Her thyroid ultrasonography revealed a parenchyma with a heterogeneous echotexture consistent with thyroiditis. No cystic or solid mass was detected. The right lobe was measured as 18x17x52 mm and left lobe as 15x17x55 mm. A pituitary MRI was requested for a differential diagnosis of the TSH-secreting pituitary adenoma (TSHoma). No pathology was detected. The alpha subunit level was normal. It was learned that the TSH levels increased at 30 and 60 minutes in the TRH stimulation test that was performed with a single TRH dose of 200 μg in another center and the TSH was suppressed by the Werner’s test. No abnormality was detected in the thyroid function tests of the first-degree relatives. Propranolol and methimazole were cut down gradually. THRβ gene mutation analysis was requested for our patient whose clinical history and laboratory results were compatible with thyroid hormone resistance. A 2-cc venous blood sample was taken from the patient in an EDTA tube and the separation of genomic DNA was performed according to the protocol of the kit. The THRβ gene ENST00000356447.8 transcript was amplified by PCR using primers within deep intronic covering exon 7, 8, 9, 10. The Sanger sequencing method was used to determine the nucleotide sequences of the DNA. Using the ProSeq and BioEdit software programs, the patient and reference genome sequences were compared and analyzed. The heterozygous missense variation NM_001128177.1(THRβ):c.1034G>A (p.Gly345Asp) was detected in protein coding region, exon 9. The variation caused a change in thyroid hormone receptor b protein amino acid 345 glycine to aspartic acid. Amino acid comparison is in Table 2 (HGMD^® Professional 2020.3). The variation was identified as rs28999970 in the dbSNP database and was reported as “pathogenic” in the ClinVar database. In the silico analysis result of the variation of NM_001128177.1(THRβ):c.1034G>A (p.Gly345Asp) “Likely Pathogenic (PM1, PM2, PM5, PP2, PP3)”, GERP Score: 5.96, DANN score: 0.9986, MutationTaster: Disease causing, SIFT: Damaging. In the ClinVar database, the NM_001128177.1(THRβ):c.1034G>A (p.Gly345Asp) variant was associated with # 188570 Thyroid Hormone Resistance, Generalized, Autosomal Dominant; GRTH (Fig. 1). Crystal structure of THRβ was obtained from protein data bank (PDB ID: 3GWS). Pymol program was used to analyze structure and produce figures (The PyMOL Molecular Graphics System, Version 2.0 Schrödinger, LLC.)

Figure 1.

THRβ gene mutation analysis.

Table 1.

Laboratory results of the patient

Laboratory Parameters (Normal Reference Ranges)	Laboratory Results
Glucose (70-100 mg/dL)	82
Creatinine (0.5-0.95 mg/dL)	0.6
AST (0-35 U/L)	15
ALT (0-35 U/L)	19
Sodium (135-145 mEq/L)	137
Potassium (3.5-5.1 mEq/L)	4.3
TSH (0.38-5.33 mIU/L)	3.053
fT4 (0.61-1.3ng/dL)	2.66
fT3 (2.8-4.7 pg/mL)	6.5
Anti-TG (0-4 IU/mL)	4.3
Anti-TPO (Anti Thyroid Peroxidase) (0-9 IU/mL)	21.2
TSH Receptor Antibody (<1.75 IU/L)	<0.30
Triglyceride (0-150 mg/dL)	110

Table 2.

Amino acid comparison

Trait	Gly (G)	Asp (D)
Amino acid name	Glycine	Aspartic acid
Polarity/charge	Non-polar	Negatively charged
pH	Neutral	Acidic
Residue weight	57	115
Hydrophobicity score	-0.4	-3.5
Hydrophilicity score	0.0	3.0
Secondary structure propensity	Strong α breaker β breaker	Weak α former strong β breaker
Grantham difference	94
MutPred likelihood of being deleterious	Very High Risk

Discussion

Thyroid hormones play an important role in the regulation of basal metabolism, maturation of the central nervous system, cardiovascular and gastrointestinal functions in our body. For normal thyroid hormone functions, the body needs a normal plasma membrane transport of thyroid hormones, a deiodination process, and thyroid hormone nuclear receptors (1). Thyroid hormone receptors (THRs) are encoded by the THRα and THRβ gene, each of which undergoes alternate splicing to generate receptor subtypes (THRα1, THRβ1 and THRβ2) with differing tissue distributions. The TR isoforms differ primarily in their N-terminal (A/B) domains. The mutant thyroid hormone receptor reduces affinity and interacts with cofactors abnormally, causing the target tissues to be resistant to thyroid hormones. THRα1 is predominant in cardiac and skeletal muscles, bones and the gastrointestinal tract; THR β1 is most abundant in the liver and kidneys and THRβ2 is discretely expressed in the hypothalamus, pituitary gland, cochlea and retina. Clinical findings in patients may show hypothyroidism, hyperthyroidism, learning disabilities, and hyperactivity according to the level of THRβ and THRα gene expression in the target tissue (3,5). Sinus tachycardia can be seen in 80% of the patients with RTH due to cardiac sensitivity (4). Our patient also had complaints of fatigue, heat intolerance and tachycardia. Despite high levels of freeT4 levels, TSH levels were not suppressed. In spite of being negative for an anti-TSH receptor antibody (TRAb), the patient was positive for anti-thyroglobulin (Anti-TG) and antithyroid peroxidase (anti-TPO) antibodies. It was reported that individuals with RTH due to THRβ mutations have an increased likelihood of autoimmune thyroid diseases (4, 6). Gavin et al. suggested that chronic TSH stimulation activates intrathyroidal lymphocytes leading to thyroid damage and autoimmune hypothyroidism in RTH (7). Marla et al. found the possibility of thyroid autoantibodies increased in individuals with RTH and reported that the relationship between them was not a coincidence (8). Therefore, we thought that the thyroid antibodies were found to be positive in our case. An MRI per pituitary protocol was followed for a differential diagnosis of TSH-secreting pituitary adenoma (TSHoma). No pathology was detected. It has been reported in the literature that patients with pituitary adenomas secreting TSH usually present with symptoms of severe hyperthyroidism and compression symptoms resulting from the tumor growth (9). A genetic examination was requested to confirm the diagnosis for the patient who was suspected of having THRβ mutation. The pathogenic variation NM_001128177.1(THRβ):c.1034G>A (p.Gly345Asp) was detected in our patient. Since it was associated with autosomal dominant inheritance, her first degree relatives were screened for the THRβ mutation. THR genes were first cloned in 1986, and later it was found that most patients with RTH have mutations in the THRβ gene (1). To date, a total of 184 THRβ gene-related mutations have been reported in the HGMD^® Professional 2020.3 database. 162 of these mutations were reported as missense / nonsense, 2 as regulatory substitutions, 9 as small deletions, 7 as small insertions / duplications, 2 as small indels, and 2 as gross deletions.

Nucleotide deletion and translocation in the RTH were determined to result from one amino acid deletion and frameshift mutations. It has been reported that proline 453 mutations replaced with arginine or histidine instead of glycine 345 in the ligand binding domain of THRβ1 reduce the binding affinity for T3 and transcriptional activity and are characterized by an autosomal dominant inheritance pattern (10-12).

Most of the pathogenic and likely pathogenic variations reported in the THRβ gene are located in 3 clusters enriched with CpG dinucleotide hotspots at the carboxy terminus of THRβ. The THRβ gene 7, 8, 9, and 10 exons encode amino acids 177 to 461 that form the carboxy terminal ligand binding domain and part of the hinge region. The pathogenic variation NM_001128177.1 (THRβ): c.1034G> A (p.Gly345Asp) located in the THRβ gene exon 9 is located in the carboxyterminal ligand-binding domain of the protein (13). We retrieved the crystal structure of THRβ in complexed with T3 (PDB ID: 3GWS) from the protein data bank (14). Inspection of structure showed that Gly345 is in the binding pocket of T3. The distance between closest atoms of T3 and Gly345 is 5.3Å. Then we mutated Gly345 to Asp using Pymol mutation tool and searched for the best-fit rotamer. Among 8 available rotamer positions, 6 of them were clashing with other amino acids. The rest 2 rotamers occupied some portion of T3 binding pocket. The distance with closest atoms of Asp and T3 is around 1.1 Å indicating that Asp mutation clashes with T3 and may destabilize its binding to THRβ. To compare occupancy of Gly and Asp amino acids on the mutated site, both of them were shown in the same figure (Fig. 2). In the THRβ gene located on the chromosome 3 of our case, we found an Allele Origin: G (germline)/T (germline)/A(germline), heterozygous missense pathogenic variation NM_001128177.1 (THRβ):c.1034G>A (p.Gly345Asp). In the treatment of RTH, the symptoms and clinical presentation of the patients should be focused on rather than the thyroid hormone levels being in the normal range. Most of the patients are followed closely without treatment. Due to the absence of active complaints from our patient, we also followed our case up without treatment. Patients presenting with hyperthyroidism should be treated symptomatically with beta blockers (3). A diagnosis of RTH requires a high clinical suspicion and a genetic mutation analysis should be requested in the case of clinical suspicion. In this way, unnecessary anti-thyroid treatment can be prevented.

Figure 2.

Occupancy of Gly and Asp amino acids on the mutated site.

Conflict of interest

The authors declare that they have no conflict of interest.