Development of Thyroid Dysfunction and Autoantibodies in Graves' Multiplex Families: An Eight-Year Follow-Up Study in Chinese Han Pedigrees

Abstract

Background:

This 8-year follow-up study is aimed at determining the relapse and development of Graves' disease (GD) and the potential risk factors that could be associated with the development of thyroid dysfunction and autoantibodies in Chinese pedigrees.

Methods:

Fifty-four Chinese Han GD pedigrees (322 members) were recruited in 2000. Forty-five pedigrees (263 members) were followed up. Their clinical and laboratory characteristics and fasting urinary iodine were measured with the same method at the two time points.

Results:

We found that the mean age for onset of GD in offspring was much younger than that of their parents (p=0.013). At baseline, the prevalence of hyperthyroidism, hypothyroidism, and subclinical hypothyroidism in first-degree relatives were 5.5%, 1.6%, and 1.1%, respectively. Individuals with thyroid dysfunction were positive for thyroid autoantibodies. The prevalence of positive thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), or TSH receptor antibody (TRAb) in the first-degree relatives with euthyroidism in these pedigrees was 18.6%, 17.4%, or 56.9%, respectively. At follow-up, individuals with positive TPOAb were at risk of developing thyroid dysfunction, whereas patients with positive TRAb had increased risk for relapse even after drug treatment. The percentage of nonsmokers with positive TPOAb and TgAb was significantly higher than that of smokers (p<0.05), but the levels of serum TRAb were significantly higher in smokers at follow-up than baseline (p<0.01).

Conclusions:

Genetic factors are crucial for the development of autoimmune thyroid disease (AITD), thyroid dysfunction, and the outcomes of Graves' patients following treatment with medicines. Although smoking was negatively associated with the presence of thyroid antibodies (TPOAb/TgAb), smoking may induce or aggravate GD.

Introduction

G raves' disease (GD) is an autoimmune thyroid disease and its development is associated with a complex interaction of susceptibility genes with environmental triggers. Genetic factors are crucial for the pathogenic process of GD and the proband-wise concordance rate for hyperthyroidism in monozygotic twins is much higher than in dizygotic twins (1). However, the concordance rate of GD is just 22%, which suggests that nongenetic factors, including smoking, infection, stress, and iodine intake, are also important in the development of GD (1,2).

Multiplex families are valuable for the evaluation of the genetic susceptibility for developing GD (3,4). However, there is no information on how the genetic factors interact with environmental factors in the development of GD in Chinese pedigrees. Our previous study examined the genetic susceptibility and the potential risk factors in 54 Chinese Han pedigrees with GD (5). In this study, we further analyzed the prevalence of thyroid autoantibodies and thyroid dysfunction in members from multiplex families 8 years later. In addition, we analyzed the potential factors affecting the natural development of thyroid diseases in first-degree relatives and the relapse of GD in patients in these multiplex families. We discussed the implication of our findings.

Subjects and Methods

Pedigrees

A total of 54 Chinese Han pedigrees (322 individuals) from Liaoning Province (located in the Northeast China) were recruited for studying factors contributing to the development of GD at outpatient service of the Department of Endocrine Diseases of our hospital in 2000. They all came from areas with adequate iodine intake. Individual pedigrees had at least one first-degree relative with GD who was diagnosed, according to the criteria of GD described below, and they were subjected to genome-wide scan (5). Individual pedigrees were ascertained through a GD proband attending and their basal levels of clinical characteristics were measured. There were 32 (59%) pedigrees with 2 affected first-degree relatives, 15 (28%) with 3, 5 (9%) with 4, and 2 (4%) with 5, respectively. Accordingly, among the 139 Graves' patients (30 males and 109 females, with the male/female ratio of 1:3.6), 81 presently had hyperthyroidism [23 males, mean age: 39 years (27–52); 58 females, mean age: 38 years (9–68)] and 58 previously had hyperthyroidism [7 males, mean age: 44 years (26–71); 51 females, mean age: 50 years (26–79)]. A total of 183 first-degree relatives participated in this study [86 males, mean age: 52 years (21–84); 97 females, mean age: 49 years (20–95)].

In 2008, 263 (81.7%) individuals from 45 (83.3%) pedigrees were interviewed and they included 114 (82.0%) Graves' patients [21 males with mean age of 39 years (26–71) and 93 females with mean age of 44 years (9–79)], except for 10 deceased patients, and 149 (81.4%) first-degree relatives [71 males with mean age of 51 years (21–81) and 78 females with mean age of 49 years (20–95)], but not the 14 deceased subjects. There was no significant difference in demographic characteristics between the individuals who were interviewed and those who were lost to follow-up. Individuals were subjected to physical examination and questionnaire, covering history of thyroid disease, past and present smoking habits, and iodized salt intake. Their blood and urine samples were collected for biochemical measurements. Written informed consent was obtained from individual subjects, and the experimental protocols were approved by the Medical Ethics Committee of China Medical University.

Individuals who smoked more than one cigarette per day were considered to be smokers, whereas those who previously smoked but stopped at least 8 years previously were considered to be ex-smokers.

Criteria of diagnosis

The diagnosis of GD was based on documented clinical and biochemical evidence of hyperthyroidism (thyrotropin [TSH] < 0.3 mIU/L, free thyroxine [FT₄] > 24.5 pmol/L, and free triiodothyronine [FT₃] > 6.3 pmol/L), diffuse goiter, and the presence of at least one of the following: positive for TSH receptor antibody tests, diffusely increased iodine-131 uptake in the thyroid gland, or the presence of exophthalmos. The diagnostic criteria for hypothyroidism were TSH > 4.8 mIU/L and FT₄ <10.3 pmol/L; for subclinical hypothyroidism were TSH > 4.8 mIU/L and FT₃ and FT₄ within the normal range; for subclinical hyperthyroidism were TSH <0.3 mIU/L and FT₃ and FT₄ within the normal range; for high concentrations of serum autoantibodies were thyroid peroxidase antibody (TPOAb) > 50 IU/mL and/or thyroglobulin antibody (TgAb) > 40 IU/mL, respectively. For diagnostic accuracy, all affected individuals were further interviewed and examined by experienced clinicians.

Measurements and methods

The baseline samples were tested in 2000 and samples from the follow-up study were examined in 2008 using the same methods and assay kits in the same laboratory by skilled technicians. All blood samples were tested for the concentrations of serum TSH, FT₃, FT₄, TPOAb, and TgAb using commercial kits (Immulite 2000 chemiluminescent immunoassay; Diagnostic Products Corp., Los Angeles, CA). The reference ranges of TSH, FT₃, FT₄, TPOAb, and TgAb were 0.3–4.8 mIU/L, 10.3–24.5 pmol/L, 2.3–6.3 pmol/L, <50 IU/mL, and <40 IU/mL, respectively. The intra-assay coefficients of variation (CV) of assays for the concentrations of serum TSH, FT₄, FT₃, TPOAb, and TgAb were 1.23%–1.38%, 3.44%–5.82%, 1.78%–7.22%, 3.51%–4.65%, and 3.86%–6.06%, respectively. The interassay CVs were 1.57%–4.93%, 6.55%–9.38%, 4.74%–9.35%, 6.22%–8.29%, and 5.82%–8.78%, respectively. The concentrations of serum TSH receptor antibody (TRAb) were measured using a M22-based TRAb ELISA (Medizym^® TRAb clone; Medipan GmbH, Dahlewitz/Berlin, Germany), with a positive cutoff value of 1.75 IU/L. The intra-assay CV was 0.89%–2.01% and the interassay CV was 0.59%–1.14%. The concentrations of serum thyroid-stimulating antibody (TSAb) and TSH-stimulating blocking antibody (TSBAb) were measured with recombinant human thyrotropin receptor (rhTSHR)-Chinese hamster ovary cell (rhTSHR-CHO cell) bioassay, as previously described (6). The intra-assay CV was <6%. Thirty blood samples tested at baseline were retested in 2008 for each thyroid hormone and for thyroid autoantibodies, and the inter-assay CVs were <9%.

Fasting morning urine samples were collected from all subjects, frozen (at −20°C), and tested in the laboratories for the measurement of iodine concentration by the colorimetric ceric ion arsenious acid ash method, based on the Sandell-Kolthoff reaction (7). The intra-assay and interassay CVs were <6.7%. Individual samples were tested in duplicate, and samples from affected and unaffected were tested simultaneously.

Statistical methods

All data were entered into an Excel database and analyzed with the SPSS statistical software package (version 11.5; SPSS Inc., Chicago, IL). Chi-square test was used for the analysis of categorical data, and Student's t-test and analysis of variance were used for the analysis of quantitative data. Risk factors were analyzed by logistic regression. In addition, the potential ascertainment biases were adjusted by the random effect model using SAS statistical software package (SAS Institute, Cary, NC) (8). A value of p<0.05 was considered statistically significant.

Results

The baseline study

Analysis of the onset age in different generations revealed that GD appeared to develop at an earlier age in the subsequent generation. Among 139 Graves' patients in 54 families, the mean ages of GD onset were 42.56±14.47 for 85 parents and 28.72±9.01 for 54 offspring, respectively. There was a significant difference in the age of onset of GD between parents with GD and offspring with GD (p=0.000), even after correction to exclude the potential selection bias (p=0.013) (8). Characterization of the thyroid autoantibodies and thyroid dysfunction indicated that 81 of 139 patients were hyperthyroid and 58 previously had hyperthyroidism. The prevalence of positive TRAb in patients with hyperthyroidism was significantly higher than those with previous hyperthyroidism (p<0.001; Table 1). Although the percentage of positive for any of the autoantibodies tested, for three autoantibodies, for either two, for TPOAb, or for TgAb in the patients who were hyperthyroid was slightly higher than that of those with previous hyperthyroidism, there was no statistically significant difference between these groups of patients.

Table 1.

The Prevalence of Patients with Thyroid Autoantibodies

	Present hyperthyroidism	Previous hyperthyroidism
Number	81	58
% of any positive autoantibodies	77/81 (95.1%)	49/58 (84.5%)
% of all positive TPOAb/TgAb/TRAb	30/81 (37.0%)	14/58 (24.1%)
% of both positive TPOAb/TgAb	40/81 (49.4%)	24/58 (41.4%)
% positive for TPOAb	60/81 (74.1%)	38/58 (65.5%)
% positive for TgAb	44/81 (54.3%)	28/58 (48.3%)
% positive for TRAb	61/81 (75.3%)^a	31/58 (53.4%)

Data are expressed as real numbers of cases and percentage at the baseline.

p<0.001 versus individuals with previous hyperthyroidism.

TPOAb, thyroid peroxidase antibody; TgAb, thyroglobulin antibody; TRAb, TSH receptor antibody.

Of the 183 first-degree relatives, there were 15 (8.2%) subjects with thyroid dysfunction [10 (5.5%) with subclinical hyperthyroidism (4 males and 6 females), 3 (1.6%) with hypothyroidism, and 2 (1.1%) with subclinical hypothyroidism]. Notably, these subjects with thyroid dysfunction were positive for thyroid autoantibodies (Table 2). However, there were 167 first-degree relatives who were euthyroid and 31 of 167 were positive for TPOAb, 29 for TgAb, and 95 for TRAb, respectively. Apparently, many first-degree relatives of GD pedigrees developed autoantibodies against thyroid antigens, but they did not have clinically overt GD. Some of them had subclinical thyroid dysfunction and others were euthyroid.

Table 2.

The Prevalence of First-Degree Relatives with Positive Thyroid Autoantibodies

	Subclinical hyperthyroidism	Hypothyroidism	Subclinical hypothyroidism	Euthyroidism
Number	10	3	2	167
% positive for TRAb	8/10 (80.0%)	3/3 (100%)	2/2 (100%)	95/167 (56.9%)
% positive for TPOAb	4/10 (40.0%)	1/3 (33.3%)	1/2 (50%)	31/167 (18.6%)
% positive for TgAb	5/10 (50.0%)	1/3 (33.3%)	0/2 (0)	29/167 (17.4%)

Data are expressed as real numbers of cases and percentage at the baseline.

The follow-up study

A total of 110 blood samples from the first-degree relatives were obtained 8 years later and subjected to laboratory examination. There were two subjects with onset of hypothyroidism, seven with subclinical hypothyroidism, one with hyperthyroidism, three with subclinical hyperthyroidism, and four with positive thyroid autoantibodies (TPOAb/TgAb), respectively. The 8-year cumulative incidence of hypothyroidism, subclinical hypothyroidism, hyperthyroidism, and positive thyroid autoantibodies (TPOAb/TgAb) were 1.8%, 6.3%, 0.9%, 2.7%, and 3.6%, respectively. Similarly, the cumulative incidence of supranormal TSH (TSH > 4.8 mIU/L) and subnormal TSH (TSH < 0.3 mIU/L) were 8.1% (9/111) and 3.6% (4/111), respectively, and the annual incidence of supranormal TSH (TSH > 4.8 mIU/L) was about 1.0%. There was no significant difference in the activity and concentrations of autoantibodies between baseline and follow-up studies (data not shown).

Analysis of the impact of thyroid autoantibodies on thyroid hormones indicated that 27 of 110 first-degree relatives were positive for autoantibodies at baseline, of which 8 (26.5%) were accompanied by thyroid dysfunction at follow-up. There were 84 without autoantibodies at baseline, and 5 (5.2%) of them displayed thyroid dysfunction at follow-up. The cumulative incidence of thyroid dysfunction was significantly higher in subjects with positive autoantibodies than those without autoantibodies (p<0.01). We further stratified these subjects into thyroid dysfunction and normofunction cohorts. In both baseline and follow-up studies, the levels of serum TPOAb and TgAb in thyroid dysfunction cohort were significantly higher than those with thyroid normofunction (p<0.01; Table 3). Similarly, the positive rates of TPOAb and TgAb in the thyroid dysfunction cohort were also significantly higher than those in the normofunction group (p<0.01). In the follow-up study, the positive rate of TRAb in the thyroid dysfunction cohort was greater than that in the thyroid normofunction cohort (p<0.05), and the activity of TSAb in the thyroid dysfunction cohort was also significantly higher than that in the thyroid normofunction group (p<0.01). More importantly, logistic regression analysis revealed that individuals with positive TPOAb (TPOAb >50 U/mL) at follow-up were at risk to develop thyroid dysfunction in these multiple pedigrees (OR=10.434, 95% CI=2.130–51.104, p=0.004).

Table 3.

Thyroid Autoantibodies in Subjects with Thyroid Normofunction and Dysfunction

	Baseline study		Follow-up study
	Thyroid dysfunction	Thyroid normofunction	Thyroid dysfunction	Thyroid normofunction
Number	13	98	12	98
TPOAb (U/mL)	63.00 (10.00–1000.00)^a	12.05 (10.00–1000.00)	165.50 (10.00–1000.00)^b	10.00 (10.00–629.65)
% of positive TPOAb	69.2%^a	20.4%	75.0%^b	14.3%
TgAb (U/mL)	43.00 (20.00–3000.00)^a	20.00 (20.00–401.85)	84.70 (20.00–3000.00)^b	20.00 (20.00–746.35)
% of positive TgAb	61.5%^a	15.3%	58.3%^b	12.2%
TRAb (IU/L)	0.61 (0.30–1.05)	0.42 (0.30–0.66)	0.77 (0.56–1.08)	0.55 (0.42–0.80)
% of positive TRAb	7.7%	4.1%	16.7%^c	1.0%
TSAb activity (%)	84.61±61.85	111.52±157.92	59.37±19.84^b	45.18±16.60
% of positive TSAb	23.1%	19.4%	0	0
TSBAb activity (%)	−18.44 (−31.81–2.15)	−10.32 (−21.93–6.99)	−34.15 (−104.35–32.70)	−47.24 (−103.30–11.93)
% of positive TSBAb	0	7.1%	0	4.1%

p<0.01 versus thyroid normofunction group in the baseline study.

p<0.01 versus thyroid normofunction group in the follow-up study.

p<0.05 versus thyroid normofunction group in the follow-up study.

TSAb, thyroid-stimulating antibody; TSBAb, TSH-stimulating blocking antibody.

Further stratification of the first-degree relatives, according to the status of smoking, found that there were no new smokers or ex-smokers. There was no significant difference in the cumulative incidence of thyroid dysfunction between smokers and nonsmokers (p>0.05). However, the basal concentrations of serum TSH and FT₄ were higher than those measured 8 years later. The positive rates of TPOAb and TgAb in nonsmokers were higher than those in smokers, and there were significant differences between smokers and nonsmokers at follow-up study (p<0.05; Table 4). The level of serum TRAb in smokers at follow-up was significantly higher than that at baseline (p<0.01).

Table 4.

Thyroid Autoantibodies and Thyroid Function in Smoking and Nonsmoking Subjects

	Baseline study		Follow-up study
	Smoker	Nonsmoker	Smoker	Nonsmoker
Number	41	70	41	69
TSH (mU/L)	1.11 (0.42–3.78)	1.41 (0.24–4.33)	1.37 (0.39–6.69)	1.54 (0.04–7.60)
FT₄ (pmol/L, x±s)	17.57±2.50	16.01±3.20	17.08±3.41	16.04±2.85
FT₃ (pmol/L, x±s)	3.98±0.94	3.98±0.78	4.06±0.85	4.20±0.66
TRAb (IU/L)	0.30 (0.30–0.30)	0.81 (0.50–1.13)	0.67 (0.37–0.67)^a	0.69 (0.69–0.80)
% of positive TPOAb	12.2%	24.3%	9.8%	26.1%^b
% of positive TgAb	14.6%	24.3%	7.3%	23.2%^b

p<0.01 versus smoker at baseline.

p<0.05 versus smoker at follow-up.

TSH, thyrotropin; FT₄, free thyroxine; FT₃, free triiodothyronine.

Further, we characterized the impact of iodine intake on thyroid dysfunction by measuring the concentrations of urinary iodine for individual subjects and stratified them according to the levels of urinary iodine (0–99 μg/L, 100–199 μg/L,…, 700–799 μg/L for each group, respectively). We found that there was no significant difference in the cumulative incidence of thyroid dysfunction in these cohorts (p>0.05).

A total of 104 patients were subjected to the follow-up study 8 years later. Among these patients, 70 patients (67.3%) were treated orally with medicine, whereas 25 patients (24.0%) received radioiodine therapy. Thirty-seven patients (52.9%) with oral medicine had relapse, whereas 3 (12.0%) with radioiodine had relapse during the previous 8 years. Further, 8 patients (7.7%) received surgery, and 4 of them (50.0%) had relapse. One did not receive any treatment. In addition, there was no significant difference in the relapse rate between smoking and nonsmoking patients following oral medicines, suggesting that smoking did not affect the relapse of GD patients following treatment with oral medicine in this population. Similarly, we found that there was no significant difference in the relapse rate among patients with different levels of urinary iodine at both baseline and follow-up studies (p>0.05). Apparently, iodine intake had no effect on the outcomes of Graves' patients following drug treatment in this population.

To determine the influence of thyroid autoantibodies on the outcomes of Graves' patients treated with drugs, we stratified 70 patients who had been treated with drugs into relapsing and nonrelapsing cohorts. We found that there was no significant difference in the positive rates and the levels of serum TPOAb and TgAb between these two groups (p>0.05; Table 5). However, the levels of serum TRAb in the relapsing cohort were significantly higher than those in the nonrelapsing cohort in both baseline study (p<0.01) and follow-up study (p=0.000). Similarly, the activity of TSAb in the relapsing cohort was significantly higher than that in the nonrelapsing cohort in both baseline (p<0.05) and follow-up studies (p=0.000), and the activity of TSAb in the relapsing cohort was also significantly higher than that in the nonrelapsing cohort in the baseline study (p<0.05; Table 5). Logistic multiple regressions indicated that a positive TRAb was associated with increased risk for the relapse in GD patients treated with oral medicine in this population (OR=2.644, 95% CI=1.276–5.478, p=0.009).

Table 5.

Thyroid Autoantibodies in Relapsing and Nonrelapsing Graves' Disease Patients

	Baseline study		Follow-up study
	Relapsing group	Nonrelapsing group	Relapsing group	Nonrelapsing group
Number	37	33	37	30
TPOAb (U/mL)	639.00 (17.70–1000.00)	219.50 (18.28–846.25)	331.00 (31.50–1000.00)	253.00 (21.93–997.00)
% of positive TPOAb	64.9%	66.7%	70.3%	70.0%
TgAb (U/mL)	64.90 (20.00–1104.50)	54.30 (20.00–156.75)	58.20 (20.00–366.00)	45.95 (20.00–282.75)
% of positive TgAb	51.4%	57.6%	56.8%	56.7%
TRAb (IU/L)	5.78 (1.10–22.05)^a	1.67 (0.64–5.32)	2.34 (1.06–7.30)^b	0.47 (0.34–0.60)
% of positive TRAb	59.5%	42.4%	56.8%	20.0%
TSAb activity (%)	793.52±1571.14^c	149.72±262.78	97.74±127.63^b	46.67±16.91
% of positive TSAb	40.5%^c	12.1%	8.1%	0
TSBAb activity (%)	9.68 (−20.84–47.27)	−4.11 (−16.97–16.09)	−40.79 (−100.90–13.46)	−32.75 (−86.03–14.10)
% of positive TSBAb	10.8%	3.0%	5.4%	6.7%

p<0.01 versus nonrelapsing cohort in the baseline study.

p=0.000 versus nonrelapsing cohort in the follow-up study.

p<0.05 versus nonrelapsing cohort in the baseline study.

Discussion

In this study, we initially examined 54 GD multiplex pedigrees and followed up for 8 years to determine the outcomes of Graves' patients, the development of thyroid autoantibodies, and thyroid dysfunction in the first-degree relatives of GD multiplex pedigrees. Eight years later, we examined 45 multiplex pedigrees with a follow-up rate of 83.3%. Analysis of all patients revealed that the mean age of onset was 42.56 for the parent generation and 28.72 for the offspring generation, supporting the notion that the onset age decreased in the subsequent generation (9). These data suggest that the genetic factors play an important role in the development of GD. Alternatively, the early onset of GD in offspring may be attributed to their growing circumstance. Further studies are needed to determine the possible factors influencing on the early onset of GD in offspring.

The iodine-induced thyroid diseases (IITD) study indicates that the prevalence of subclinical hyperthyroidism, hypothyroidism, and subclinical hypothyroidism in a general population is 3.1%, 1.0%, and 3.2%, respectively (10). We found that the prevalence of subclinical hyperthyroidism, hypothyroidism, and subclinical hypothyroidism were 5.5%, 1.6%, and 1.1% in 54 GD pedigrees, respectively. The prevalence of thyroid dysfunction in the first-degree relatives of GD patients was not different from that in the general population of similar ethnic background and iodine status. The prevalence of thyroid autoantibodies in individuals with subclinical hyperthyroidism, hypothyroidism, and subclinical hypothyroidism is 33.9%, 76.9%, and 34.5%, respectively, in the general population (10). However, we found that individual first-degree relatives with thyroid dysfunction were all positive for thyroid autoantibodies. Further, the prevalence of euthyroid subjects with positive TPOAb or TgAb in a general population is 10.3% and 9.1% (11), whereas the prevalence of euthyroid first-degree relatives with positive TPOAb or TgAb was 18.6% and 17.4%. Apparently, the prevalence of thyroid autoantibodies in the first-degree relatives of GD patients was significantly higher than that in the general population of similar ethnic background and iodine status. These data further suggest that the genetic factors are crucial for the development of thyroid dysfunction in Chinese.

Previous studies in a European cohort reported that the 5-year incidence of hyperthyroidism or hypothyroidism in subjects with a family history is high (12,13), but we found that the incidence of hypothyroidism, subclinical hypothyroidism, hyperthyroidism, and subclinical hyperthyroidism in first-degree relatives were 0.2%, 0.8%, 0.1%, and 0.3%, which was not remarkably different from that of the general population in IITD (10). The incidence of supranormal TSH (TSH > 4.8 mIU/L) was 1.0%, higher than 0.4% from the IITD study (11). Notably, the prevalence of TPOAb or TgAb positivity in subjects with supranormal TSH was 77.8%, which was significantly higher than that in the general population (11). Logistic regression analysis revealed that a positive TPOAb was associated with increased risk for development of thyroid dysfunction in first-degree relatives of GD multiplex pedigrees. These data suggest that the TPOAb is not only a marker for the presence of underlying thyroid autoimmunity, but also may be valuable for the prognosis of thyroid dysfunction in individuals with a family history of GD.

We found that the relapse rate in individual GD patients with positive TRAb was significantly higher than that of those without it. Notably, many patients responded well to routine treatment and their thyroid function recovered. The high rate of relapsing in TRAb-positive patients suggests that treatment did not effectively control the thyroid-specific autoimmunity, particularly in those TRAb-positive patients, leading to relapse. Conceivably, the positive TRAb may be used as a biomarker for prognosis in terms of relapse of GD in Chinese patients.

Previous studies on how smoking affects thyroid function and thyroid-specific autoimmunity remain controversial (14,15). Effraimidis et al. found that smoking was negatively associated with the development of thyroid antibodies (15), whereas others indicated that smoking did not affect the pathogenic process of Hashimoto's disease (14). Further, low levels of serum TSH and high levels of FT₄ and FT₃ are usually detected in heavy smokers (16). Similarly, we detected low levels of serum TSH and high levels of FT₄ in smoking first-degree relatives. Interestingly, we found that the positive rate of TPOAb and TgAb in nonsmoking relatives was significantly higher than that of smoking relatives in both baseline and follow-up studies. However, the levels of serum TRAb in smoking relatives in the follow-up study were significantly higher than that of the baseline study. It is possible that smoking may not initiate the development of thyroid specific autoimmunity, but can accelerate the autoimmune process in those who have already developed thyroid specific autoimmunity, leading to the development of GD in Chinese.

We recognize that this study has potential limitations. There were only two time points for an 8-year follow-up study, which limited the observation of dynamic changes in the thyroid function and autoimmunity in those members. We failed to collect information from all members, increasing possibility of selective nonresponse and loss to follow-up. We are interested in further following up this population closely.

In summary, this is the first report on the natural outcomes of GD in members from GD multiplex families over an 8-year period. Our data indicate that genetic factors are crucial for the development of thyroid dysfunction and GD in Chinese as well as the outcomes of GD patients following treatment with routine medicines. Our findings suggest that smoking may be associated with the acceleration of thyroid-specific autoimmunity in those who have developed autoimmunity. Further, the TPOAb is associated with increased risk for the development of thyroid dysfunction in first-degree relatives and TRAb influences on the relapsing of GD in GD patients. Therefore, TPOAb may be valuable for prognosis of thyroid dysfunction and TRAb for the relapsing of GD in Chinese.

Footnotes

Acknowledgments

The authors are grateful to all the patients and their pedigree members who participated in this study and the physicians who referred pedigrees and verified diagnoses. The study was supported by grants from the National Natural Science Foundation of China (30370680, 36670966), and a grant from the China Guanghua Foundation (grant number: China Guanghua Foundation 2007-02).

Disclosure Statement

The authors acknowledge that each of them has participated sufficiently in this work to take public responsibility for its content. The authors state that they have no potential conflict of interest.

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