BACKGROUND
Thyroid hormone plays an important role in keeping bodily temperature stable when the outside temperature changes. In the cold, an increase in thyroid hormone leads to an increase in basal metabolic rate and heat generation. This is reversed in the heat. Thyroid hormone levels are delicately regulated through the hypothalamus-pituitarythyroid axis; thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates the pituitary gland to secrete thyrotropin-stimulating hormone (TSH), which stimulates the thyroid gland to produce thyroid hormone. TRH and TSH productions are, in turn, regulated through the feedback from thyroid hormone levels in blood.

Previous studies have shown that TSH levels vary from season to season, with higher TSH levels seen in colder seasons and lower TSH levels in warmer seasons. This finding likely reflects the body’s adaptation to the cold by increasing basal metabolic rate and body heat. Other studies reported changes in different aspects of thyroid function regulation in the hypothalamus-pituitary-thyroid axis when exposed to different temperatures. However, exactly how thyroid function is regulated in response to changing temperature remains unclear. The authors of this study aimed to assess changes in thyroid function in changing temperature using the national database of Korea, where temperatures can vary widely between seasons.

THE FULL ARTICLE TITLE
Sung J & Kim JH. Association between ambient temperature and thyroid-stimulating hormone and free thyroxine levels in Korean euthyroid adults. Environ Res 2024;262(Pt 2):119918; doi: 10.1016/j. envres.2024.119918; PMID:39237021.

SUMMARY OF THE STUDY
In this study, 4659 adults with normal thyroid function and no history of thyroid disease were selected from the Korea National Health and Nutrition Examination Survey database between 2013 and 2015. Blood TSH and free thyroxine (FT4) levels and participants’ characteristics were collected. The outside temperature on the day of the blood draw was determined using the national database on air quality. Associations between temperature and thyroid function were assessed.

The average age of participants (55% men and 45% women) was 44 years old. The average TSH level was 2.4 mIU/L with overall adequate iodine status. The average temperature throughout the year was 13.6°C (56.5 °F), with a range of –13.9 to 30.9 °C (7.0 to 87.6 °F). A decrease in TSH levels was seen with increase in temperature. Exposure to cold temperature (-6.7 to -2.8 °C or 19.9 to 27.0 °F) was associated with an increase in TSH levels by 3.6-4.5%. Exposure to hot temperature (27.4 to 29.8 °C or 81.3 to 85.6 °F) was associated with a decrease in TSH levels by 3.1-3.7%. There was no significant association between temperature and FT4 levels in the cold. However, at temperatures above 8.9 °C (48.0 °F), increasing temperature was associated with decreasing FT4 levels.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
In Korean adults with normal thyroid function, increasing outside temperature was associated with decreasing TSH levels. This finding suggests that TSH levels can be affected by exposure to different ambient temperature. The findings of this study confirm similar reports of TSH variation with seasonal temperature variation, although the absolute degree of change in TSH was small. A longitudinal study following individuals through different temperature exposure would be helpful in further understanding changes in thyroid hormone regulation in response to temperature. Global warming is of increasing concern, with recent reports of the highest global temperature in over 100,000 years. It would be important to understand how changes in temperature can potentially affect the regulatory mechanisms of thyroid hormones, as patients with thyroid dysfunction or patient taking thyroid hormone replacement may not be able to respond well to quick changes in temperature. These patients may be more susceptible to heat-related health problems.

— Sun Y. Lee, MD

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