Abstract
Background Various changes in thyroid hormones (TH) and thyroid-stimulating hormone (TSH) level were observed in different seasons among euthyroid and hypothyroid subjects living in areas with an extreme temperature difference between summer and winter. Objectives This study aims at finding the effect of temperate climate on the seasonal variations of TSH and TH in euthyroid and subclinical hypothyroidism (SCH) subjects and at evaluating if the test season has an effect on the number of subjects diagnosed as SCH. It basically focuses on the relation of different components of climate with TH and TSH. Method In a prospective study on 152 healthy (euthyroid) volunteers and 25 SCH subjects, the serum hormone levels (TSH, FT4, and FT3) were measured in both the summer and winter seasons and correlated with all the climate components using Pearson's correlation coefficient. The effect of duration of outdoor exposure on hormone levels was compared using a paired sample t-test (P < 0.05). Results Small but statistically significant increased FT3 level and decreased FT4 level were observed during the winter season in euthyroid and SCH subjects, respectively. There was a significant negative correlation between FT3 and FT3/FT4 ratio with temperature and sunshine duration and a positive correlation with humidity and atmospheric pressure. A positive correlation was found between FT4 and sunshine duration. Conclusion The climate components contributed to the slight variance in hormone levels in different seasons, and the effect was mostly on peripheral conversion of FT4 to FT3 rather than the pituitary-thyroid axis leading to slightly higher FT3 in winter. Seasonal variation does not affect the diagnosis of SCH cases.
Highlights
The hypothalamic-pituitary-thyroid (HPT) axis, known as thyroid homeostasis, is a part of the neuroendocrine system, responsible for the biosynthesis and secretion of thyroid hormones (TH) thyroxine (T4) and triiodothyronine (T3). This is regulated by the release of thyroid-stimulating hormone (TSH) from the anterior pituitary, which in turn is stimulated by the release of thyrotropin releasing hormone (TRH) from the hypothalamus
We found that the variations in the climatic components were associated with the TSH and TH levels in euthyroid subjects, with no effect on subclinical hypothyroidism (SCH) subjects
Increased sunshine duration and temperature during summer decrease the activity of this enzyme slightly. Each climatic component such as humidity, sunshine duration, temperature, cloud cover, and atmospheric pressure has its share on the slight variance of hormone levels found in different seasons, the effect being mainly on FT3 and FT3/free thyroid hormones T4 (FT4) ratio
Summary
The hypothalamic-pituitary-thyroid (HPT) axis, known as thyroid homeostasis, is a part of the neuroendocrine system, responsible for the biosynthesis and secretion of thyroid hormones (TH) thyroxine (T4) and triiodothyronine (T3). Most of the studies showed an increase in TSH in winter [6, 9] or with prolonged exposure to cold temperatures [7]; a few studies exhibited an increase in T3 and/or T4 [7, 10], while a decrease in the TH levels was observed [6,7,8] The cause of these seasonal changes in thyroid hormone and TSH level varies; it could be due to a International Journal of Endocrinology centrally mediated response of the hypothalamic-pituitarythyroid axis [8, 11], change in thyroidal secretion [12], change in protein binding [13], ambient luminosity [6], or peripheral metabolism of thyroid hormone during different seasons [14]. Seasonal variation does not affect the diagnosis of SCH cases
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