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.

Keywords: Iodine; Korea; Thyrotropin; Thyroxine.

Fig. 1. Serum thyroid stimulating hormone (TSH) and free thyroxine (fT₄) levels by age and sex. (A) Geometric mean TSH levels (mIU/L) by age. (B) Mean fT₄ levels (ng/dL) by age. A U-shaped association was found between serum TSH levels and age. TSH levels were significantly higher in women than in men (P<0.001). Serum fT₄ levels significantly decreased with age (P for trend <0.0001), and were significantly higher in men than in women (P<0.0001). Adapted from Park et al. [6].

Fig. 2. Relationship of urinary iodine concentration (UIC) with serum thyroid stimulating hormone (TSH) levels. A U-shaped curve was found between age and serum TSH levels, with lower levels in middle-aged adults and higher levels in younger and older age groups. A U-shaped curve was also detected for the relationship between age and UIC. Therefore, changes in serum TSH levels with age may be influenced by changes in UIC in the Korean population. Adapted from Park et al. [6].

Fig. 3. Urinary iodine concentration (UIC, µg/L) by age and sex. According to the World Health Organization recommendations, only 23% of subjects were in the adequate range (UIC, 100 to 199 µg/L), and 65% were classified as having an intake that was above requirements (UIC 200 to 299 µg/L) or excessive (UIC ≥300 µg/L). However, 12% had insufficient iodine intake (UIC <100 µg/L). The median UIC was higher in school-aged children (6 to 12 years) and in the above 70 years age group than in other age groups. Adapted from Park et al. [6].