All subjects in this study came from a community in Chengdu (1 of 10 cities selected for epidemiological investigations of thyroid disease in China). Fifteen hundred residents who had lived in Chengdu for at least 5 years and were more than 18 years old were selected using a stratified cluster sampling technique. The response rate was 80.3%. The 1,205 subjects that completed the investigation were included in this analysis. The exclusion criteria for the study were as follows: (1) Women who were pregnant or less than one year postpartum; (2) subjects taking glucocorticoids, dopamine, or dobutamine; (3) subjects taking antiepileptic drugs (phenytoin, carbamazepine, or others); (4) subjects suffering from adrenocortical insufficiency, renal insufficiency, or other serious systemic disease or chronic wasting disease; or (5) a person receiving amiodarone or an iodine-containing contrast agent within the past 6 months.
All the participants were asked to complete a self-assessment questionnaire that included demographic data, reproductive history, history of smoking and previous thyroid disease, and family history of thyroid disease. Height, weight, and blood pressure were measured in all participants. Fasting blood samples were collected and centrifuged at 3,000 revolutions per minute for 5 minutes. Sera were decanted for storage at −20°C until assayed. Fasting urine specimens from 80 school-age children from a school in the community were collected and frozen (−20°C).
A thyroid ultrasonography was performed with a 7.5 MHz (LOGIQ 500, GE Healthcare, China). Thyroid volume was calculated by multiplying the thickness, width, length, by a corrective factor (0.479) for each lobe [10]. The diameter of thyroid nodules was measured and recorded. The thyroid echo pattern was classified as isoecho, hyperecho, hypoecho, and heterogeneous echo.
The study was approved by the ethics committee of West China Hospital, Sichuan University. Participants signed a consent form after being informed verbally about the study.
Laboratory methods
Thyroid function and thyroid autoantibodies were measured using chemiluminescence immunoassay kits (Roche Kit, Cobas-e601 analyzer). Intraassay and interassay coefficient of variation were all less than 5%. Thyroperoxidase autoantibody (TPOAb), thyroglobulin autoantibody (TgAb), and thyroid stimulating hormone (TSH) were measured in all participants. If TSH was <0.71 mU/L, the free thyroxine (FT4) and free triiodothyronine (FT3) were also measured in the same sample. If TSH was >6.25 m U/L, then only FT4 was measured. Serum selenium was determined by hydride generation atomic fluorescence spectrometry (HG-AFS) using a method described elsewhere [11]. Serum was digested in a mixture of concentrated HNO3 and HC1O4 (V: V = 4:1) first and then determined by HG-AFS. During the process of digestion, the temperature was kept below 180°C to prevent selenium from volatilizing. Digestion was continued at 180°C until the solution became clear. The solutions of HCL (V:V = 1:20) and KBH4 (M:V = 1.5:100) diluted by deionized water were used as the carrier liquid and reducing agent, respectively. The median urinary iodine was measured by As-Ce catalytic chromatography. Laboratory reference values were 12 to 22 pmol/L for FT4, 3.1 to 6.8 pmol/L for FT3, 0.71 to 6.25 mU/L for TSH, <34 IU/mL for TPOAb, and <115 IU/mL for TgAb.
Diagnostic criteria for thyroid disease
Clinical hyperthyroidism (overt thyrotoxicosis) was defined as a TSH < 0.71 mU/L and an FT4 >22 pmol/L, and/or a TSH <0.71 mU/L and an FT3 > 6.8 pmol/L. Subclinical hyperthyroidism (subclinical thyrotoxicosis) was based on a normal FT4 and FT3, and a TSH <0.71 mU/L. Clinical hypothyroidism (overt hypothyroidism) was defined as a TSH >6.25 mU/L and FT4 <12 pmol/L. Subclinical hypothyroidism was based on a normal FT4 and TSH >6.25 mU/L.
Goiter was defined as a thyroid volume exceeding 18.8 mL for males and 14.4 mL for females. The cut-off levels were derived from the mean (+2 SD) thyroid volume in 597 subjects (315 males and 282 females) without thyroid dysfunction, without a previous thyroid disease, without a family history of thyroid disease, without positive thyroid autoantibodies, and without goiter or nodules on ultrasonography [12].
Statistical methods
Data processing was done with the SPSS version 16.0 (Chicago, SPSS Inc.). Laboratory values were reported as mean ± SD, percentage, median, and interquartile range, where approximately. Normality of the data distribution was assessed with the Kolmogorov-Smirnov test. For continuous variables, nonparametric statistics (Mann–Whitney or Kruskal-Wallis), and parametric statistics (t test) were all used, as appropriate. Group differences in the numbers of subjects were analyzed using the chi-square test. The relationship between serum selenium concentration and thyroid volume was investigated by simple and multivariate linear regression, with log-transformed thyroid volume as the dependent variable. The relationship between selenium status and thyroid goiter was examined by univariate and multivariate logistical regression. Apart from serum selenium levels, the following variables were included in the models: age, gender, smoking, body mass index (BMI), TSH concentrations, thyroid nodules, and thyroid echogenicity. The level of significance was set at 5%.
The serum selenium concentration and thyroid volume distribution in the 1,205 subjects were skewed, but showed a normal distribution after log transformation (Figure 1).