Relation of Different Components of Climate with Human Pituitary-Thyroid Axis and FT3/FT4 Ratio: A Study on Euthyroid and SCH Subjects in Two Different Seasons

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.

Seasonal Changes of Thyroid Function Parameters in Women of Reproductive Age Between 2012 and 2018: A Retrospective, Observational, Single-Center Study

“…In the longitudinal analysis of 181 subjects, no significant differences were detected in the thyroid parameters between summer and winter. Consistent with our observations, previous studies in healthy adults have shown that TSH levels decreases in summer-spring, while it increases in autumn-winter (6,7,15,(19)(20)(21)(22)(23)(24)(25). The seasonal variation of TSH between winter and summer reached an absolute difference of more than 10%-15% in a multinational study of Belgian, British, and Japanese residents (21).…”

“…The monthly average temperatures range from -11.2°C in January to 24.6°C in July ( 14 ). Therefore, the distinct summer and winter climates in this region make Shenyang an appropriate location to investigate the effect of climatic variations on thyroid hormone levels in its population ( 15 ). In this study, “spring” was defined as being from March to May, “summer” from June to August, “autumn” from September to November, and “winter” from December to February.…”

Within-Person Variation in Serum Thyrotropin Concentrations: Main Sources, Potential Underlying Biological Mechanisms, and Clinical Implications

“…Furthermore, although the 10 (aged 32 to 66 years) ( 53 ) and 7 (aged 27 to 66 years) ( 54 ) primary hypothyroid patients in two longitudinal Japanese studies received constant doses of levothyroxine (L-T4), their unstimulated TSH levels and TSH levels after TRH stimulation were higher in winter compared to summer. In contrast, no significant difference was found in mean (SD) TSH levels measured in 25 subclinical hypothyroid patients with a mean (SD) age of 36.44 (14.82) years in winter [6.04 (1.17) mU/L] compared to summer [6.64 (1.60) mU/L] in Iraq ( 50 ).…”

“…Likewise, no significant seasonal difference was present in a small subset of 159 subjects of this study in which TSH was measured twice in the same year ( 49 ). In 152 Iraqi euthyroid subjects with a mean (SD) age of 32.24 (12.05) years who had their TSH levels measured in both summer and winter seasons, no significant difference was found between summer [mean (SD) of 2.34 (1.25) mU/L] and winter [2.25 (1.25) mU/L] measurements ( 50 ). Andersen et al found 8% higher serum TSH concentrations in autumn and winter than in spring and summer using monthly blood samples in 15 healthy Danish men with an age range of 24–53 years, but this difference was not statistically significant ( 51 ).…”

Association of exposure to ambient air pollution with thyroid function during pregnancy

“…[39][40][41] We did not adjust for season because, to our knowledge, no evidence exists on the association of season of pregnancy with free T 4 or TSH levels (despite associations with air pollution). 42,43 Pregnant women who were included in the analysis were different from those excluded because of missing data on exposure and outcome (eTable 3 in the Supplement). To address the selective nonresponse arising from these differences, we used inverse probability weighting.…”