Coldness or Darkness? Which Places Greater Stress on the Thyroid? Seasonal Changes in Thyroid-Stimulating Hormone and Thyroid Hormones.

Although seasonal changes were suggested to be among the many factors that affect thyroid functions, this issue is still controversial. In this study, we aimed to investigate the possible relationship between seasonal changes and thyroid function. We retrospectively scanned all thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) values checked in our hospital between 2019 and 2023. Using the big data approach, we examined the relationship between TSH and thyroid hormones and monthly and seasonal recorded climatic changes, particularly the duration of daylight and temperature. A total of 195,534 serum samples were analyzed for TSH, 195,491 for FT3, and 195,487 for FT4. There were significant differences in the values of TSH, FT3, and FT4 between months ( = 0.001 for TSH, < 0.001 for FT3 and FT4). The months with the highest levels of TSH, FT3, and FT4 values were January, December, and June, while the months with the lowest levels were July, May, and March-April, respectively. The differences between the maximum and minimum median values were 14.5% for TSH, 4.9% for FT3, and 5.7% for FT4. From January to August, as the temperature rose, there was a decrease in TSH values. Between September and December, as the temperature decreased, an increase in TSH was observed. This study revealed that TSH, FT3, and FT4 show seasonal variability and change in temperature is an important factor that plays role in this variability. It would be appropriate to take these changes into consideration when interpreting thyroid function tests.