Abstract
Background: Loss-of-function mutations in the thyrotropin receptor (TSHR) gene lead to resistance to TSH (RTSH) presenting with either congenital hypothyroidism (CH) or subclinical hypothyroidism (SCH). Despite several reports of patients with TSHR mutations, data on the long-term outcome of this condition are limited, and no consensus exists on the need for hormone replacement therapy. The aim of the present study was to assess the long-term outcome in children and adolescents with RTSH due to TSHR mutations.
Methods: The TSHR gene was sequenced in 94 subjects (aged 3 days–21 years) with either nonautoimmune SCH or CH with RTSH.
Results: Twenty-seven subjects (29%) carried mutations in TSHR. Fifteen infants were identified by neonatal screening, and the other 79 patients were detected in the process of testing for various other conditions or because of family occurrence of thyroid test abnormalities. Six different mutations were identified: c.484C>G (p.P162A), c.202C>T (p.P68S), c.790C>T (p.P264S), c.269A>C (p.Q90P), c.1957C>G (p.L653V), and c.1347C>T (p.R450C). Twelve subjects were homozygous, three were compound heterozygous, and 12 were heterozygous. Mean serum TSH levels at diagnosis and at last visit were significantly higher in patients with TSHR mutations than in those without mutations (29.04 vs. 14.15, p=0.002; 31.73 vs. 6.19, p<0.0001, respectively). Homozygous patients had a more severe phenotype (TSH 53.6 vs. 9.24, p<0.0001). Mean serum free thyroxine (fT4) levels at the last visit were significantly lower than at the first visit in the homozygous individuals (p=0.05) for a follow-up period of as long as 11 years. Heterozygous subjects had only mild hyperthyrotropinemia with stable TSH levels. However, homozygous subjects showed a trend toward increased TSH and decreased fT4 with time.
Conclusion: SCH in heterozygotes with TSHR mutations is a stable compensated condition with an appropriately adjusted set point for pituitary–thyroid feedback that does not require replacement therapy. However, homozygous subjects, with incompletely compensated SCH, show reduced fT4 levels over time and may require levothyroxine treatment. Replacement therapy should be considered on an individual basis, and long-term follow up is recommended.
Introduction
Resistance to thyrotropin (RTSH) is characterized by elevated serum TSH levels, normal or low triiodothyronine (T3) and/or thyroxine (T4) concentrations, and a eutopic, normal, or hypoplastic thyroid gland. Loss-of-function mutations in the TSH receptor (TSHR) gene lead to RTSH characterized by either congenital hypothyroidism (CH) or subclinical hypothyroidism (SCH) (1–6).
The definition of SCH is a serum TSH concentration above the upper limit of the reference range, with fT4 and fT3 levels within the reference range. It suggests a compensated state of primary thyroid failure requiring increased levels of TSH to maintain normal levels of thyroid hormones (7).
To date, about 60 different loss-of-function mutations in the TSHR have been described (4–6). The degree of hyposensitivity to TSH depends on the type and location of the TSHR mutation and whether the patient is homozygous or heterozygous. More severe loss of TSHR function manifests as CH, whereas mild or heterozygous mutations present as SCH in childhood or adulthood.
The prevalence of inactivating TSHR mutations differs among populations, based on the selected clinical criteria, and on whether the subjects present with CH or SCH. A prevalence of 4.3% biallelic TSHR mutations has been reported among Japanese infants with CH (8), whereas a high prevalence of 29% has been found among Italian children with nonautoimmune SCH (9).
Despite several reports of subjects with TSHR mutations, limited data are available on the long-term outcome of this condition, and no consensus exists on the need for supplemental thyroid hormone therapy. In a cross-sectional analysis of an extended family with 33 members having TSHR mutations, it has previously been shown that TSH levels are stable with age, suggesting permanently compensated RTSH with an appropriately adjusted set point for pituitary–thyroid feedback (10).
In the current study, the long-term outcome of 94 patients presenting with RTSH characteristics, with or without TSHR mutations, for up to 20 years was assessed. This large sampling and long-term observation enabled the delineation of the clinical course of SCH in children.
Subjects and Methods
Patients
Ninety-four subjects were included in the study (54 males) with a mean age at presentation of 52 months (range 3 days–21 years). Selection of subjects for TSHR sequencing was based on the following criteria: (a) subjects with TSH levels above the upper limit of the reference range (>4.2 mIU/L) in at least two separate determinations with normal thyroid hormone levels; (b) infants identified by national neonatal screening for hypothyroidism in whom persistent elevation of TSH with normal or slightly below normal range fT4 values were observed after replacement treatment was withdrawn (replacement therapy is withdrawn at the age of two to three years, as per the routine management of infants with CH); (c) eutopic thyroid gland of normal size or hypoplastic on 99pertechnetate (99Tc) scan, or lack of a thyroid gland on 99Tc scan but a normal or hypoplastic gland by ultrasonography (ultrasonographic imaging was performed when the 99Tc scan showed lack of a gland with measured serum thyroglobulin); and (d) siblings of subjects with SCH or identified as having TSHR mutations. Subjects were excluded under the following criteria: (a) positive autoantibodies to thyroid peroxidase (TPO) and thyroglobulin (TG); and (b) known genetic conditions or chromosomal abnormalities. All patients were followed at 3- to 12-month intervals for as many as 20 years, and thyroid function tests were performed at three- to six-month intervals. Thyroid morphology was evaluated by scintigraphy in all patients with CH. All of the clinical parameters were collected retrospectively from the medical records, and results of biochemical tests were collected from the health-insurance companies' computerized systems. All data presented in this study were obtained from patients while off levothyroxine (LT4) therapy.
The phenotypes of some of these subjects have been previously described (10,11), but in the present study, the aim was to assess the long-term outcome in these individuals along with additional subjects with RTSH characteristics.
Written informed consent was obtained from all participants. This study was approved by the Ethics Committee of Ha'Emek Medical Center, the Genetics Committee of the Israeli Ministry of Health, and the Institutional Review Board of the University of Chicago.
Hormonal analysis
TSH, fT4, FT3, and TG and TPO antibodies were measured by direct automated chemiluminescent immunoradiometric assays using ADVIA Centaur (Bayer Corporation, Tarrytown, NY). Serum TG levels were measured by a direct automated chemiluminescent immunoradiometric assay using Immulite 2000 (Siemens, Llanberis, Gwynedd, United Kingdom).
National neonatal screening
The Israeli National Newborn Screening Program for CH was implemented in 1987. Blood samples are collected by heel puncture 48–72 h after birth. Between 1987 and 2006, the Israeli national newborn screening laboratory performed the tests using DPC (Los Angeles, CA) radioimmunoassay. Since 2006, measurements have been made by PerkinElmer B065-112 AutoDELFIA neonatal total thyroxine (TT4) and B032-312 AutoDELFIA neonatal TSH, both using time-resolved fluoroimmunoassays. The Israeli CH screening program is based on a TT4 test followed by confirmatory TSH test. Therefore, when the level of TT4 is below the daily 10th percentile, TSH is measured. TSH values >20 mIU/L are considered indicative of primary CH. Neonates with abnormal screening results are referred to medical centers. Negative screening results were retrieved retrospectively from the national computerized database.
Molecular analysis
Genomic DNA was isolated from peripheral leucocytes by standard methods. All coding regions of the TSHR gene were amplified (sequences available on request). Sequencing reactions for both DNA strands were performed with the BigDye Terminator Cycle Sequencing Ready Reaction Kit (Applied Biosystems, Foster City, CA) and analyzed with an automatic sequencer (Applied Biosystems). The TSHR coding regions of the 94 subjects enrolled in the study were sequenced.
Statistical analysis
Statistical analyses were performed with SAS v9.2 statistic software package (Cary, NC). Groups were compared using the Student's t-test. When the Student's t-test assumptions were not valid for continuous variables, the Wilcoxon two-sample test was used. The chi-square test (or Fisher's exact test) was used for categorical variables. Significance was set at p<0.05. Scatter plots were generated for TSH and fT4 levels versus age. Loess smoothing was used to express trends with age. The Wilcoxon sign rank test was used to determine the difference in TSH and fT4 levels between the first and last measurements in patients with TSHR mutations.
Results
Ninety-four subjects were included in the study. Of these, 88 were of Arab-Muslim origin with a high incidence of consanguinity, and six were of Jewish origin. Seventy-nine subjects had SCH, and 15 were identified by national neonatal screening as having CH (Table 1). TSHR mutations were identified in 27 subjects (29%). Mean serum TSH levels were significantly higher both at diagnosis (p=0.002) and at the last visit (p<0.0001) in patients with TSHR mutations compared to those without mutations (Table 1). Mean serum fT4 concentrations did not differ at presentation but were slightly lower at the last visit in subjects with TSHR mutations (p=0.0195; Table 1). The difference in TSH concentration (ΔTSH) between the first and last visit was significantly greater in subjects with TSHR mutations (p<0.002), but the ΔfT4 did not differ (Table 1). The evolution of TSH and fT4 values over the years, for both groups, is shown in Figure 1.
Table 1.
Characteristics of All Patients (94 Subjects) and Comparisons Between Subjects With and Without TSHR Gene Mutations
| Parameter | Total (mean ± SDS) [median] (range) | TSHR mutation | Without TSHR mutations | p-Value | Normal range |
|---|---|---|---|---|---|
| Sex (female/male) | 40(42%)/54(58%) | 13(48%)/14(52%) | 27(40%)/40(60%) | 0.4861 | |
| Arab/Jewish | 88 (94%)/6 (6%) | 26 (96%)/1(4%) | 61 (91%)/6 (9%) | 0.1772 | |
| Consanguinity (%) | 35 (39%) | 16 (64%) | 19 (29%) | 0.0024* | |
| Age at presentation (years) | 4.4 ± 4.9 [2.2] (0.01–21.8) |
5.1 ± 5.4 [3.65] (0.01–21.8) |
4.2 ± 4.7 [2.01] (0.01–16.7) |
0.4906 | |
| SCH detected later in life | 79 (84%) | 22 (81%) | 57 (85%) | 0.1727 | |
| Neonatal screening | 15 (16%) | 5 (19%) | 10 (15%) | ||
| TSH (mIU/L) at presentation | 18.47 ± 23.72 [9.0] (4.22–134) |
29.04 ± 27.09 [13.5] (4.22–91) |
14.15 ± 20.92 [8.33] (5.12–134) |
0.0020* | 0.27–4.2 |
| fT4 (pmol/L) at presentation | 15.78 ± 4.89 [15.1] (0.52–46.49) |
15.11 ± 3.11 [14.87] (9.5–24.81) |
15.29 ± 3.14 [15.4] (0.52–46.49 |
0.2682 | 12.0–22.0 |
| TSH (mIU/L) at last visit | 13.57 ± 23.17 [5.45] (2.32–138.51) |
31.73 ± 36.43 [13.97] (3.69–138.51) |
6.19 ± 6.37 [4.83] (2.32–47.83) |
<0.0001* | 0.27–4.2 |
| fT4 (pmol/L) at last visit | 14.51 ± 2.58 [14.45] (6.8–23.2) |
13.45 ± 2.43 [13.52] (9.0–17.09) |
14.96 ± 2.52 [14.82] (6.8–23.2) |
0.0195* | 12.0–22.0 |
| ΔTSH | −0.24 ± 17.71 [−1.63] (−65.93–97.26) |
5.8 ± 26.95 [2.17] (−61.44–97.26) |
−2.62 ± 11.84 [−2.43] (−65.93–41.62) |
0.0016* | |
| ΔfT4 | −1.55 ± 5.51 [−0.46] (−32.5–8.53) |
−1.54 ± 3.32 [−1.6] (−12.35–2.89) |
−1.55 ± 6.33[−0.3] (−32.5–8.53) |
0.3945 | |
| LT4 therapy (no. patients) | 17 (18%) | 9 (34%) | 8 (11.76%) | 0.0347* | |
| TSH (mIU/L) screening | 37.66 ± 52.32 [21.5] (1.9–250) |
38.06 ± 20.8 [40.0] (12.6–67) |
37.51 ± 60.93 [18.95] (1.9–250) |
NS | >24 |
| TT4 (μg/dL) screening | 10.08 ± 3.48 [9.8] (2.8–18.92) |
9.32 ± 4.05 [9.6] (2.8–17.47) |
10.43 ± 3.21 [9.85] (4.7–18.92) |
NS | >10 |
p < 0.05.
ΔTSH, TSH at last visit – TSH at presentation; ΔfT4, fT4 at last visit – fT4 at presentation.
TSHR, thyrotropin receptor; SCH, subclinical hypothyroidism; fT4, free thyroxine; LT4, levothyroxine; NS, not significant.
FIG. 1.
Thyrotropin (TSH) and free thyroxine (fT4) concentrations as a function of age in subjects without (left panel) and with (right panel) thyrotropin receptor (TSHR) gene mutations. Dashed lines represent age-related reference ranges (15).
Six different mutations were identified: c.484C>G (p.P162A), c.202C>T (p.P68S), c.790C>T (p.P264S), c.269A>C (p.Q90P), c.1957C>G (p.L653V), and c.1347C>T (p.R450C). Five of these have been previously described (12–14), and one was novel (Table 2). Twelve subjects were homozygous, three were compound heterozygous, and 12 were heterozygous. Twelve single-nucleotide polymorphisms (SNPs) were identified, consisting of 10 known and two unknown variants (Supplementary Table S1; Supplementary Data are available online at www.liebertpub.com/thy). All but one of the subjects harboring TSHR mutations were of Arab-Muslim origin, belonging to four extended families. Age at first presentation ranged from 3 days to 21 years, with five infants identified by neonatal screening. The others were detected in the process of testing for various other conditions or because of family occurrence of thyroid test abnormalities. In the five infants who were detected by neonatal screening (four homozygous and one heterozygous), repeated thyroid tests in the authors' laboratory, prior to initiation of LT4 therapy, revealed normal fT4 levels in all except case 9, and four were treated with LT4. Thyroid tests at two years of age, after LT4 withdrawal, showed SCH, and only case 9 had persistent hypothyroidism (Table 3).
Table 2.
Summary of Identified TSHR Gene Mutations
Table 3.
Characteristics of Subjects with TSHR Gene Mutations
| Neonatal screening | At presentation | Last visit | Imaging | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Case | Family | M/F | TSH (mIU/L) | TT4 (μg/dL) | Age (months) | Cause for preforming thyroid tests | TSH (mIU/L) | fT4 (pmol/L) | Age (years) | TSH (mIU/L) | fT4 (pmol/L) | LT4 therapy | Genotype | 99TC | US |
| 1a | 1 | F | 40 | 4.7 | 0.5 | Screening | 30.75 | 13.77 | 6.66 | 19.79b | 16.35b | 0.46m | L653V/L653V | Normal | ND |
| 2c | 1 | M | ND | 12.9 | 151 | Familial | 61.25 | 9.52 | 13 | 39 | 11.45 | — | L653V/L653V | Normal | 3 mm echogenic area |
| 3 | 1 | M | 43 | 5.7 | 1.0 | Screening | 72.45 | 16.39 | 6.0 | 14.63b | 16.71b | 1m | L653V/L653V | Low trapping | ND |
| 4 | 1 | M | NA | NA | 0.5 | TF | 10.32 | 20.89 | 1.6 | 14.19 | 15.45 | — | L653V/W | ND | ND |
| 5 | 2 | F | NA | 13.5 | 43.2 | Familial | 59.97 | 17.54 | 3.8 | 49.71 | 17.09 | — | P162A/P162A | ND | ND |
| 6 | 2 | F | NA | 10.4 | 74.4 | Familial | 32.9 | 14.74 | 6.3 | 42.76 | 15.71 | — | P162A/P162A | ND | ND |
| 7 | 2 | F | 21.5 | 5.4 | 126 | TF | 17.53 | 14.4 | 12.25 | 9.29 | 16.29 | 11.8y | P162A/P162A | ND | ND |
| 8 | 3 | F | ND | 11.8 | 44.4 | TF | 5.42 | 13.93 | 13.11 | 7.14 | 16.82 | — | P68S/W | ND | Normal |
| 9 | 4 | M | 67 | 2.8 | 0.7 | Screening | 68.3 | 5.34 | 7.3 | 6.86b | 12.98b | 0.7m | Q90P+P264S/Q90P+P264S | Low trapping | ND |
| 10 | 4 | F | NA | NA | 18 | TF | 13.5 | 18.58 | 5.4 | 13.79b | 16.84b | 18m | Q90P+P264S/W | Normal | Normal |
| 11 | 4 | M | 12.6 | 13.0 | 0.3 | Familial | 11.84 | 15.5 | 6 | 14.14 | 13.9 | — | Q90P+P264S/W | ND | ND |
| 12 | 4 | M | ND | 17.47 | 0.1 | Familial | 16.3 | 24.8 | 0.33 | 6.26 | 15.65 | — | Q90P+P264S/W | ND | ND |
| 13 | 4 | F | ND | 10.9 | 1 | Neonatal jaundice | 9.0 | 16.6 | 11.3 | 13.6 | 12.85 | 1m | Q90P+P264S/W | Normal | Normal |
| 14 | 5 | M | NA | NA | 47.3 | Short stature | 10.54 | 15.1 | 9.6 | 8.1 | 13.9 | — | L653V/P68S | ||
| 15d,e | 5 | M | NA | NA | 252 | Familial | 84 | 10.6 | 29 | 97.3 | 9.0 | 29y | L653V/L653V | Normal | Normal |
| 16 | 5 | F | NA | NA | 31.2 | TF | 5.88 | 13.3 | 3.75 | 3.69 | 13.5 | — | P68S/W | ND | ND |
| 17d,f | 5 | F | NA | 9.6 | 75.6 | Familial | 41.25 | 12.6 | 14 | 138.5 | 9.0 | 14y | L653V/L653V | Normal | Normal |
| 18d | 5 | F | NA | NA | 123 | Palpitation | 52.5 | 13.3 | 19 | 100.2 | 9.9 | 19y | L653V/L653V | Enlarged lobe | Normal |
| 19 | 5 | F | NA | NA | 84 | Viteligo | 6.45 | 17 | 9.5 | 6.45 | 17.5 | — | L653V/P68S | ND | ND |
| 20 | 5 | M | NA | NA | 133 | Familial | 6.02 | NA | 15.0 | 4.05 | ND | — | L653V/W | ND | ND |
| 21 | 5 | M | 17 | TF | 8.83 | 11.6 | 6.83 | 7.7 | 12.6 | — | R450C/W | ND | ND | ||
| 22 | 5 | M | ND | 10.2 | 51 | Familial | 31.78 | 14.07 | 10.1 | 5.7b | 15.1b | 5.33y | L653V/L653V | Normal | Normal |
| 23 | 5 | M | 21.3 | 7.92 | 0.1 | Screening | 10.59 | 14.2 | 1.66 | 17.65 | 14.7 | — | L653V/W | ND | Normal |
| 24 | 5 | M | 61 | 8.8 | 0.7 | Screening | 91.0 | 15.8 | 6.1 | 0.65b | 18.7b | 0.7m | L653V/L653V | Low uptake | Normal |
| 25 | 5 | M | NA | NA | 96 | TF | 12.67 | 14.2 | 11.0 | 12.39 | 13.1 | — | L653V/P68S | ND | Normal |
| 26 | 5 | M | NA | NA | 28 | Familial | 4.22 | 15.9 | ND | ND | ND | — | R450C/W | ND | ND |
| 27 | 5 | F | NA | NA | 83 | TF | 7.7 | 12.4 | 12.33 | 8.41 | 9.3 | — | L653V/W | ND | ND |
| Reference range | <20 | >10 | 0.35–5.0 | 10.8–18.7 | 0.35–5.0 | 10.8–18.7 | |||||||||
Patients 9 and 11 were heterozygous for mutation of TPO gene (C493S).
Nephrotic syndrome.
LT4 therapy.
Refused to take medication.
LT4 therapy was initiated at last visit.
Positive TGab at last visit.
Convulsive disorder.
TT4, total thyroxine; TF, thyroid function; NA, not available; ND, not done.
Since the Israeli national screening is based on TT4 measurements, test results from the 22 subjects who were not detected by the screening were retrieved retrospectively from the national database. In these cases with normal TT4 levels, TSH was not tested. Mean screening TSH and TT4 levels did not differ between subjects with or without TSHR mutations. Thyroid glands were of normal size or hypoplastic but in a normal location in the subjects with TSHR mutations, as demonstrated by 99TC scan or by ultrasonography. Only one patient (case 18) had an enlarged lobe (Table 3).
All homozygous and 30% of the heterozygous subjects were consanguineous. Homozygotes had a more severe phenotype manifested by higher TSH levels at diagnosis and at last visit (mean TSH 53.6 vs. 9.24, p<0.0001; 62.07 vs. 9.79, p=0.0005, respectively). Despite the significantly higher serum TSH concentrations, mean serum fT4 levels did not differ between homozygotes and heterozygotes (13.96 vs. 16.27 at diagnosis, p=0.175; 12.53 vs. 14.35 at last visit, p=0.1006, respectively). Compound heterozygous subjects had one severe mutation and one very mild one (L653V/P68S) presenting with a moderate phenotype, reflecting the weak functional effect of the P68S mutation (9,11). They were therefore not included in the comparison between the homozygotes and heterozygotes. Heterozygous subjects had mild hyperthyrotropinemia with TSH levels that remained stable over time (Fig. 2). In homozygous subjects, a slight decrease in fT4 with the passage of time was accompanied by a slight increase in TSH (Fig. 2). In some of these individuals, fT4 levels declined below the lower normal limit for age. The mean serum fT4 level in homozygous subjects was significantly lower at the last visit compared to the first visit (p=0.05), but did not differ in the heterozygous subjects in a follow-up period of 11 years. All subjects tested negative for TPO and TG antibodies at presentation and in repeated testing until the last visit in all but one subject (case 15) who tested positive for TG antibodies at her last visit. Three patients (patients 15, 17, and 18; Table 3) carrying homozygous TSHR mutations, in whom an elevated TSH persisted for as long as nine years, underwent a brain magnetic resonance imaging scan that demonstrated normal-sized pituitary glands. Eleven subjects were treated with LT4 based on individual physicians' decisions. However, neither clinical evidence of hypothyroidism nor biochemical parameters suggestive of hypothyroidism (elevated creatine phosphokinase or hyperlipidemia) were observed.
FIG. 2.
TSH and fT4 concentrations as a function of age in subjects heterozygous (left panel) and homozygous (right panel) for TSHR gene mutations. Dashed lines represent age-related reference ranges (15).
Discussion
A high prevalence of TSHR mutations was identified among 94 subjects with RTSH. Nicolleti et al. (9) reported similar findings among children with nonautoimmune SCH in Italy, but in that study, all of individuals were heterozygous, whereas in the present cohort, 16 subjects had either homozygous or compound heterozygous mutations. Contrary to the Italian cohort, which enrolled only SCH subjects, both SCH and CH children with RTSH characteristics were included in the current study. Moreover, the present cohort included family members of the index cases, which could result in selection bias. A prevalence of 4.3% biallelic TSHR mutations was shown among 134 Japanese infants with CH (8), and 12% of both one mutant allele and biallelic mutations were found in 42 subjects with nonautoimmune isolated hyperthyrotropinemia in Italy; all displayed familial occurrence (12). Camilot et al. (13) identified 13 patients with heterozygous mutations (11%) out of 116 children with asymptomatic euthyroid hyperthyrotropinemia. A prevalence of 0.6% for carriers of the TSHR mutation W546X was identified in Welsh euthyroid individuals (14).
The high prevalence of TSHR mutations within the present cohort is explained by the high occurrence of consanguinity in the population (38%), which lives in the same geographical area, and by the selective enrolment criteria. Furthermore, the identification of only six different mutations is explained by a founder effect in the extended families (10,11). Whereas in the five previously described mutations, functional analysis revealed that these sequence changes result in TSHR inactivation (3,10,11) for the novel identified sequence variant c.1347C>T (p.R450C), no functional analysis was performed. Nevertheless, 67 subjects with characteristics of RTSH were negative for TSHR mutations, suggesting additional etiologies for nonautoimmune SCH. Therefore, further molecular analysis is indicated to delineate the specific etiologies for SCH in those cases. A nonsignificant trend toward decreasing fT4 levels within the normal ranges for age was observed in the subjects without TSHR mutations, with a few cases having fT4 levels below the reference range.
Patients were found to have SCH following routine thyroid function tests for various conditions that retrospectively were not related to clinical hypothyroidism, and for familial occurrence of abnormal thyroid function tests. Only five infants carrying TSHR mutations were detected by the neonatal screening because the Israeli national screening is based on the measure of TT4, and only infants with low TT4 had their TSH analyzed further. Among the patients with CH, only one patient had persistent hypothyroidism. The other 22 patients were diagnosed later due to various complaints or because of familial occurrence of thyroid abnormalities. Those patients with TSHR mutations had significantly higher TSH levels compared to those without mutations. However, fT4 levels only differed at the last visit, and they were within the reference range in both groups. When the group with TSHR mutations was subdivided into those with one mutant allele and those with a biallelic mutation, the homozygotes were seen to have a more severe phenotype, manifested by significantly higher serum TSH values. Despite this difference in TSH, fT4 levels did not differ between the two subgroups.
In a previous study, it was shown by cross-sectional analysis of 33 affected family members, including parents and grandparents carrying the same two TSHR mutations (L653V and P68S), that heterozygous individuals have stable TSH levels that do not require medical therapy (10). In the present study, the long-term outcome over an 11-year period is summarized in individuals identified at 3 days to 21 years as being affected by RTSH by retrospectively analyzing the TSH and fT4 values in each individual. As in the authors' previous report, individuals heterozygous for TSHR mutations had stable TSH levels when results from the first and last visit were compared. The trend toward decreasing fT4 values over the years is explained by the normal age-related decline in thyroid hormone levels in childhood (15). In contrast, in the homozygous subgroup, the TSH values showed a significantly increasing trend with concomitant decline in fT4 levels, which in a few subjects reached values below the reference range. These findings indicate incompletely compensated hypothyroidism in individuals carrying biallelic TSHR mutations that may necessitate LT4 therapy. There is only one longitudinal study of subjects with TSHR mutations, which describes five subjects homozygous for the R450H TSHR mutation who presented with congenital hyperthyrotropinemia. Of the five individuals, one developed frank hypothyroidism during adolescence (16), supporting the need for long-term follow-up, especially in homozygotes.
The decision to treat children with SCH is a matter of debate (17–20). The risk of developing overt hypothyroidism in an adult population with SCH has been estimated at 2–4.3% per year, with a higher occurrence in patients with evidence of autoimmune thyroid disease and increased TSH at presentation (17). In children, most of the subjects with SCH remain euthyroid over time, and therefore careful follow-up, rather than empirical treatment, has been suggested (21,22). It is commonly accepted that children with a TSH>10 mIU/L should be treated, even if the fT4 level is within the reference range, whereas those with a TSH between 4.5 and 10 mIU/L with thyroid autoantibodies should be followed up with repeated thyroid function tests, but not treated (23). This this approach was followed, and the target of treatment was to maintain a serum TSH in the reference range. However, in the present cohort, some subjects with a TSH>20 mIU/L were not treated, either because the patients refused to take medication or because they were lost to follow-up. Despite the fact that some subjects were not treated for as long as nine years, no pituitary hyperplasia was demonstrated, suggesting that pituitary imaging is not required in this context. Autoimmune thyroid disease concomitant with TSHR mutations may lead to overt hypothyroidism (24). This is most likely due to the concomitant thyroid gland destruction by the autoimmune process, which does not occur in RTSH alone. Nevertheless, the benefit of LT4 therapy has been questioned, and some studies have shown no difference in metabolic parameters between treated and untreated subjects (17). This study is the first long-term follow-up of children with nonautoimmune SCH that provides evidence of the clinical courses. SCH in subjects heterozygous for loss-of-function TSHR mutations is a stable compensated condition with an appropriately adjusted set point for pituitary–thyroid feedback that does not require replacement therapy. However, in homozygous subjects, incompletely compensated SCH may develop over time, which may necessitate LT4 therapy. In view of the controversy that still exists today regarding LT4 therapy in SCH and the present findings, it is recommended that the decision to initiate therapy be considered on an individual basis, taking into account the benefits and possible side effects.
Supplementary Material
Acknowledgments
We thank Camille Vainstein for professional language editing. This work was supported in part by combined grants from the Rappaport Faculty of Medicine, Technion, Haifa, and Ha'Emek Medical Center, Afula, Israel (to Y.T.R.), as well as grants R37DK15070 and UL1TR000430 from the National Institutes of Health (to S.R.).
Author Disclosure Statement
No competing financial interests exist.
References
- 1.Refetoff S. 2003. Resistance to thyrotropin. J Endocrinol Invest 26:770–779 [DOI] [PubMed] [Google Scholar]
- 2.Sunthornthepvarakul T, Gottschalk ME, Hayashi Y, Refetoff S. 1995. Resistance to thyrotropin caused by mutations in the thyrotropin-receptor gene. N Eng J Med 332:155–160 [DOI] [PubMed] [Google Scholar]
- 3.de Roux N, Misrahi M, Brouner R, Houang M, Carel JC, Granier M, Le Bouc Y, Ghinea N, Boumedienne A, Toublanc JE, Milgrom E. 1996. Four families with loss of function mutations of the thyrotropin receptor. J Clin Endocrinol Metab 81:4229–4235 [DOI] [PubMed] [Google Scholar]
- 4.Cassio A, Nicoletti A, Rizzello A, Zazzetta E, Bal M, Baldazzi L. 2013. Current loss-of-function mutations in the thyrotropin receptor gene: when to investigate, clinical effects, and treatment. J Clin Res Pediatr Endocrinol 5:29–39 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Biebermann H, Winkler F, Kleinau G. 2010. Genetic defects, thyroid growth and malfunctions of the TSHR in pediatric patients. Frontiers Biosci 15:913–933 [DOI] [PubMed] [Google Scholar]
- 6.Tenenbaum-Rakover Y. 2012. The clinical spectrum of thyrotropin receptor gene (TSHR) mutations. In: Springer D. (ed) Hypothyroidism—Influences and Treatments. InTech; Available at www.intechopen.com/articles/show/title/the-clinical-spectrum-of-tsh-receptor-tshr-mutations (accessed February8, 2012) [Google Scholar]
- 7.Tenenbaum-Rakover Y. 2013. Approach to subclinical hypothyroidism. In: Potluková E. (ed) Current Topics in Hypothyroidism with Focus on Development. InTech; Available at www.intechopen.com/books/current-topics-in-hypothyroidism-with-focus-on-development/approach-to-subclinical-hypothyroidism-in-children (accessed February2, 2013) [Google Scholar]
- 8.Narumi S, Muroya K, Abe Y, Yasui M, Asakura Y, Adachi M, Hasegawa T. 2009. TSHR mutations as a cause of congenital hypothyroidism in Japan: a population-based genetic epidemiology study. J Clin Endocrinol Metab 94:1317–1323 [DOI] [PubMed] [Google Scholar]
- 9.Nicoletti A, Bal M, De Marco G, Baldazzi L, Agretti P, Menabò S, Ballarini E, Cicognani A, Tonacchera M, Cassio A. 2009. Thyrotropin-stimulating hormone receptor gene analysis in pediatric patients with non-autoimmune subclinical hypothyroidism. J Clin Endocrinol Metab 94:4187–4194 [DOI] [PubMed] [Google Scholar]
- 10.Tenenbaum-Rakover Y, Grasberger H, Mamanasiri S, Ringkananont U, Montanelli L, Barkoff MS, Dahood AM, Refetoff S. 2009. Loss-of-function mutations in the thyrotropin receptor gene as a major determinant of hyperthyrotropinemia in a consanguineous community. J Clin Endocrinol Metab 94:1706–1712 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Sriphrapradang C, Tenenbaum-Rakover Y, Weiss M, Barkoff MS, Admoni O, Kawthar D, Caltabiano G, Pardo L, Dumitrescu AM, Refetoff S. 2011. The coexistence of a novel inactivating mutant thyrotropin receptor allele with two thyroid peroxidase mutations: a genotype–phenotype correlation. J Clin Endocrinol Metab 96:E1001–E1006 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Tonacchera M, Perri A, De Marco G, Agretti P, Banco ME, Di Cosmo C, Grasso L, Vitti P, Chiovato L, Pinchera A. 2004. Low prevalence of thyrotropin receptor mutations in a large series of subjects with sporadic and familial nonautoimmune subclinical hypothyroidism. J Clin Endocrinol Metab 89:5787–5793 [DOI] [PubMed] [Google Scholar]
- 13.Camilot M, Teofoli F, Gandini A, Franceschi R, Rapa A, Corrias A, Bona G, Radetti G, Tato L. 2005. Thyrotropin receptor gene mutations and TSH resistance: variable expressivity in the heterozygotes. Clin Endocrinol 63:146–151 [DOI] [PubMed] [Google Scholar]
- 14.Jordan N, Williams N, Gregory JW, Evans C, Owen M, Ludgate M. 2003. The W546X mutation of the thyrotropin receptor gene: potential major contributor to thyroid dysfunction in a Caucasian population. J Clin Endocrinol Metab 88:1002–1005 [DOI] [PubMed] [Google Scholar]
- 15.Hübner U, Englisch C, Werkmann H, Butz H, Georgs T, Zabransky S, Herrmann W. 2002. Continuous age-dependent reference ranges for thyroid hormones in neonates, infants, children and adolescents established using the ADVIA Centaur Analyzer. Clin Chem Lab Med 40:1040–1047 [DOI] [PubMed] [Google Scholar]
- 16.Mizuno H, Kanda K, Sugiyama Y, Imamine H, Ito T, Kato I, Togari H, Kamoda T, Onigata K. 2009. Longitudinal evaluation of patients with a homozygous R450H mutation of the TSH receptor gene. Horm Res 71:318–323 [DOI] [PubMed] [Google Scholar]
- 17.Biondi B, Cooper DS. 2008. The clinical significance of subclinical thyroid dysfunction. Endocr Rev 29:76–131 [DOI] [PubMed] [Google Scholar]
- 18.Vanderpump MP, Tunbridge WM. 2002. Epidemiology and prevention of clinical and subclinical hypothyroidism. Thyroid 12:839–847 [DOI] [PubMed] [Google Scholar]
- 19.Surks MI, Ortiz E, Daniels GH, Sawin CT, Col NF, Cobin RH, Franklyn JA, Hershman JM, Burman KD, Denke MA, Gorman C, Cooper RS, Weissman NJ. 2004. Subclinical thyroid disease: scientific review and guidelines for diagnosis and management. JAMA 291:228–238 [DOI] [PubMed] [Google Scholar]
- 20.Pearce SHS, Vaisman M, Wemesu LJ. 2011. Management of subclinical hypothyroidism: the thyrologists' view. Eur Thyroid J 1:45–50 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Moore DC. 1996. Natural course of “subclinical” hypothyroidism in childhood and adolescence. Arch Pediatr Adolesc Med 150:293–297 [DOI] [PubMed] [Google Scholar]
- 22.Radetti G, Gottardi E, Bona G, Corrias A, Salardi S, Loche S, Study Group for Thyroid Diseases of the Italian Society for Pediatric Endocrinology and Diabetes (SIEDP/ISPED) 2006. The natural history of euthyroid Hashimoto's thyroiditis in children. J Pediatr 149:827–832 [DOI] [PubMed] [Google Scholar]
- 23.Gopalakrishnan S, Chugh PK, Chhillar M, Ambardar VK, Sahoo M, Sankar R. 2008. Goitrous autoimmune thyroiditis in a pediatric population: a longitudinal study. Pediatrics 122:e670–e674 [DOI] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.