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Context Seasonal variation in thyroid function, especially serum FT3 and FT4 levels, in healthy subjects remains unclear. Methods We examined thyroid function, including serum FT3 and FT4 levels, in healthy Japanese subjects using data of more than 7,000 health check-up participants, and applied the analysis of means with transformed ranks (ANOMTR) to compare each month. In addition, we reviewed reports published in the last two decades. Results The median serum TSH level was the highest in January (1.61 mIU/L), and the lowest in May (1.16 mIU/L). ANOMTR revealed that serum TSH levels are high in winter and low in summer. Conversely, the median serum FT3 level was higher in July than in other months, and ANOMTR plot demonstrated serum FT3 levels to be significantly higher in summer and lower in winter. In contrast, serum FT4 levels were more consistent throughout the year, but statistically, those in February and March, October, and November were higher than those in other months. ANOMTR revealed variations in serum FT4 levels to be small through the year but biphasic. Conclusions Taken together with previous reports, our study demonstrated seasonal changes in the serum TSH levels to be high in winter in the northern hemisphere; however, the serum FT3 differed among countries, and those of Japanese, an iodine-sufficient country, were high in summer. In contrast, FT4 levels were more consistent. These changes should be taken into account to precisely evaluate thyroid function.

Seasonal Variation in Thyroid Function in Over 7,000 Healthy Subjects in an Iodine-sufficient Area and Literature Review