Update on Thyroid Hormone Levels and Thyroid Dysfunction in the Korean Population Based on Data from the Korea National Health and Nutrition Examination Survey VI (2013 to 2015)

Abstract

In 2017, the first Korean nationwide data on serum thyroid stimulating hormone (TSH) levels, serum free thyroxine (fT4) levels, and urinary iodine concentration (UIC) were published based on a population of 7,061 Koreans who participated in the Korea National Health and Nutrition Examination Survey VI. The mean TSH level was 2.16 mIU/L, with a reference interval of 0.59 to 7.03 mIU/L (men 2.09 mIU/L, women 2.24 mIU/L, P<0.001). A U-shaped association was found between serum TSH levels and age. The mean fT4 level was 1.25 ng/dL, and its reference interval was 0.92 to 1.60 ng/dL (men 1.29 ng/dL, women 1.20 ng/dL, P<0.0001). Serum fT4 levels decreased with age (P for trend <0.0001). Serum thyroid peroxidase antibody (TPOAb) was detected in 7.30% of participants (men 4.33%, women 10.62%). TPOAb titers tended to increase with age, and were higher in women than in men. The median UIC was 294 μg/L, and UIC showed a U-shaped relationship with age. According to the World Health Organization recommendations, only 23% of participants were in the adequate range of iodine intake, while 65% were in the above requirements or excessive, and 12% in insufficient. The prevalence of overt hyperthyroidism and hypothyroidism in Koreans was 0.34% to 0.54% and 0.73% to 1.43%, respectively.

Association between Iodine Intake, Thyroid Function, and Papillary Thyroid Cancer: A Case-Control Study

“…The UIC of 300 μg/L corresponds to iodine intake of 450 μg/day, which corresponds to six cups of plain yogurt, eight cups of milk, and 20 slices of white bread, [ 13 , 18 , 19 ]. The thyroid function levels of each type of thyroid function indicators were classified into ‘low,’ ‘normal,’ and ‘high’ groups according to ‘Seoul National University Hospital Clinical Lab’ reference range as clinical cutpoints [ 20 ] and two groups according to population cutpoints [ 21 ]. The population cutpoints were the mean value of each hormone (TSH, 2.16 μIU/mL; free T4, 1.25 ng/dL), derived from data the Korea National Health and Nutrition Examination Survey, 2013 to 2015 (KNHANES) [ 21 ].…”

“…The thyroid function levels of each type of thyroid function indicators were classified into ‘low,’ ‘normal,’ and ‘high’ groups according to ‘Seoul National University Hospital Clinical Lab’ reference range as clinical cutpoints [ 20 ] and two groups according to population cutpoints [ 21 ]. The population cutpoints were the mean value of each hormone (TSH, 2.16 μIU/mL; free T4, 1.25 ng/dL), derived from data the Korea National Health and Nutrition Examination Survey, 2013 to 2015 (KNHANES) [ 21 ]. Because T3 information was not available from KNHANES, the population cutpoints for T3 were derived from the controls in this study.…”

“… PTC, papillary thyroid cancer; PTMC, papillary thyroid microcarcinoma; OR, odds ratio; CI, confidence interval; TSH, thyroid stimulating hormone; T4, thyroxine; T3, triiodothyronine. a Adjusted for age, sex, education level, family history of cancer, past history of chronic disease (dyslipidemia and benign thyroid disease), and total energy intake; b PTMC with 5 mm≤ tumor size <10 mm; c Clinical cutpoint and population cutpoint for each thyroid hormone are referenced in the [ 20 ], and the [ 21 ] mean value based on the data from KNHANE VI (Korea National Health and Nutrition Examination Survey in 2013–2015), respectively; d Population cutpoint of T3 (ng/mL) derived from controls. …”

“… c Clinical cutpoint and population cutpoint for each thyroid hormone are referenced in the [ 20 ], and the [ 21 ] mean value based on the data from KNHANE VI (Korea National Health and Nutrition Examination Survey in 2013–2015), respectively; …”

Subclinical Hypothyroidism: Prevalence, Health Impact, and Treatment Landscape

“…Therefore, using a single ULN for TSH for all age groups may lead to the misclassification of elderly individuals as having sHypo, which in turn could impact prevalence estimates. Importantly, age-related differences vary by country; the TSH median and 97.5th percentile increased progressively with age in the United States population [ 18 ], but in Koreans, the TSH and age graph was U-shaped (higher in younger and older subjects) [ 19 , 20 ]. Certain other factors (e.g., pregnancy, obesity, and dwelling conditions) can also impact the TSH reference range.…”