![]() ![]() Serum TSH was measured using an E-TSH kit (Roche Diagnostics, Mannheim, Germany). Serum TSH, FT4, and TPOAb concentrations were measured by an electrochemiluminescence immunoassay. Life style risk factors (i.e., smoking status) were based on self-reporting. Written informed consent was obtained from all participants or parents/guardians.ĭata on demographic characteristics, personal medical history, and family history of thyroid diseases were collected by interview during the survey. The study protocol was approved by the Institutional Review Board of the Korea Centers for Disease Control and Prevention. The reference population was selected from the disease-free population after exclusion of those with positive TPOAb, defined as ≥ 34.0 IU/ml, as provided by the kit manufacturer. The disease-free population was selected from the total population after exclusion of individuals with known thyroid disease, family history of thyroid dysfunction, and current pregnancy. ![]() To define precise reference intervals of thyroid hormones, disease-free and reference populations were established. The participants responded to a questionnaire regarding family history of thyroid disease, personal history of thyroid disease, and drugs that could affect thyroid hormone level. In this sample, urinary iodine (UI) in a spot urine sample was measured in 6,564 individuals. This subsample of KNHANES VI consisted of 7,061 individuals aged 10 years and older, weighted to represent the total Korean population. In this data set, approximately 2,400 individuals (about one-third of the total sample) were selected each year between 20 using stratified subsampling and underwent measurements for serum TSH, FT4, and TPOAb. KNHANES VI (2013–2015) was conducted using a stratified, multistage, clustered probability sampling design. The KNHANES surveys were designed to obtain national estimates of the health and nutritional status of Koreans from 1998 and are an ongoing surveillance system. In addition, we evaluated the association between iodine intake by age and TSH in this population. In this article, we provide the age- and gender-specific reference intervals of TSH and FT4 from a large, nationwide, stratified dataset (KNHANES VI) from an iodine-replete area. Thus, we sought to determine the reference intervals of TSH and FT4 from nationwide representative data of Korea. KNHANES VI (2013–2015) introduced serum TSH, FT4, and TPOAb and thyroid disease-related items in the questionnaires. The Korea National Health and Nutrition Examination Surveys (KNHANES) is a nationwide cross-sectional survey to obtain national estimates of the health and nutritional status of Koreans from 1998 and is an ongoing surveillance system. Although age and iodine are very important determinants in determination of the reference intervals of TSH and other thyroid hormone levels, there have been no such measurements performed according to age group and gender in Korea, where iodine intake is quite different from Western countries. Above-requirement or excessive iodine intake according to World Health Organization (WHO) epidemiological criteria was reported in preschool children as well as adults. Korea is known to be an iodine-replete area due to the popular nationwide dietary intake of iodine-rich seaweeds such as kelp and laver. If we use the same reference interval of TSH in older patients as is younger patients, there is a chance of over-diagnosis of subclinical thyroid disease, which could lead to unnecessary treatment with levothyroxine in the older population. Many studies have shown that the median and upper limit of TSH increase with age. Current National Academy of Clinical Biochemistry (NACB) guidelines recommend that the reference intervals of TSH should be established from the 95% confidence limits of the log-transformed values of at least 120 thyroid peroxidase antibody (TPOAb)-negative, ambulatory, euthyroid subjects without goiter or family history of thyroid dysfunction. However, the reference intervals of TSH are affected by many factors such as age, gender, ethnicity, iodine intake, body mass index (BMI), smoking, and the presence of thyroid autoantibody. In this clinical scenario, establishing a reference interval of TSH is critical for the diagnosis of subclinical thyroid functional disorders. With the increased awareness of thyroid disorders and health check-ups, there are increased incidences of subclinical hypo- and hyperthyroidism. Subclinical thyroid disease comprises subclinical hypothyroidism, defined as elevated TSH with normal free thyroxine (FT4), and subclinical hyperthyroidism, with decreased TSH and normal FT4. Serum thyroid-stimulating hormone (TSH) is the most sensitive marker to evaluate individual thyroid functional status and is used as a screening test for identifying subjects with thyroid dysfunction.
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