Background: Establishment of the reference interval of thyrotropin (TSH) is critical in the diagnosis of thyroid dysfunction and is affected by age, gender, iodine nutrition, and ethnicity. The aim of this study was to set up the age- and gender-specific reference intervals of TSH and evaluate its association with iodine status.
Methods: We analyzed data from the Korea National Health and Nutrition Examination Survey VI that measured serum TSH, free thyroxin (FT4), and thyroid peroxidase antibody from 7,061 individuals. Age- and gender-specific reference intervals were established from 95 % confidence limits from the 2.5 to 97.5 percentile of TSH (log-transformed) and FT4 in reference populations. In addition, its association with iodine status was evaluated by 2 methods: ‘estimated urinary iodine excretion (UIE)’ from spot urine samples and ‘estimated dietary iodine intakes’ from 24-hour dietary recalls.
Results: The overall reference interval of TSH was 0.59 - 7.03 mU/L, showing U-shape changes with age and higher in females. The reference interval of FT4 was 0.92 - 1.60 ng/dL, showing continuous decreases with age after 20 years. The median of UIC, estimated UIE and estimated dietary iodine intakes were 293.9 (interquartile range, 156.5 - 683.6) μg/L, 244.2 (129.1 - 570.8) μg/day and 286.4 (160.3 - 829.1) μg/day, respectively, showing an overall excessive iodine status in Koreans. There was a weak but highly significant positive correlation between TSH and iodine status (estimated UIE, P < 0.01; estimated dietary iodine intakes, P = 0.02).
Conclusions: The reference interval of TSH in Korea, where iodine intake is more than adequate, was higher than those of Western populations, reflecting the paramount importance of iodine intake on thyroid function.