The pituitary-thyroid axis in healthy men living under subarctic climatological conditions

Abstract

In order to evaluate the effects of climatic factors on the secretion of thyroid hormones and TSH in a high latitude population, we have taken serum and urine samples from 20 healthy men from northern Finland (67 -68 N) every 2 months for a period of 14 months. Serum free triiodothyronine (T 3 ) levels were lower in February than in August (3·9 vs 4·4 pmol/l, P<0·05) and TSH levels were higher in December than during other months (2·1 vs 1·5-1·7 mU/l, P<0·01). Serum total and free thyroxine (T 4 ), total T 3 and reverse T 3 levels and urinary T 4 levels were unchanged. Urinary T 3 levels were significantly higher in winter than in summer. Serum free T 3 correlated highly significantly with the outdoor temperature integrated backwards weekly for 7-56 days (r=0·26 for 1-56 days) from the day when the blood samples were taken. Serum TSH did not show any significant correlation with the thyroid hormones or with the integrated temperature of the previous days, but it did show an inverse and significant correlation (r= 0·31) with the ambient luminosity integrated backwards for 7 days from the day when the blood sample was taken. The gradually increasing correlation between outdoor temperatures and serum free T 3 suggests that the disposal of thyroid hormones is accelerated in winter, leading to low serum free T 3 levels and a high urinary free T 3 excretion. Since there was no correlation between thyroid hormones and serum TSH, the feedback mechanism between TSH and thyroid hormones may not be the only contributing factor, and other factors such as ambient luminosity may at least partly determine serum TSH in these conditions. Also urinary free T 3 appears to be a novel and non-invasive indicator for thyroid physiology.

Psychoneuroendocrine effects of combined thyroxine and triiodothyronine versus tyrosine during prolonged Antarctic residence

“…Physiologically, and presumably as part of hypothermic cold adaptation (5), polar expeditioners in the Antarctic have been found to experience a fall in body temperature (8) and an apparent 70% increase in daily energy requirements (3,9). A similar seasonal pattern of alteration in thyroid hormones has also been observed in residents living in the Arctic (10).…”

Differential expression and seasonal modulation of VGF peptides in sheep pituitary

“…In the same species, the pituitary pars tuberalis is probably involved in stimulating prolactin secretion (Graham et al 2002), while prolactin may be involved in seasonal inhibition of gonadotropin release via a paracrine intrapituitary mechanism (Tortonese et al 1998), probably also involving dopamine (Gregory et al 2004). Conversely, thyrotropes can be stimulated in response to reduced ambient luminosity, in apparent connection with a higher need for heat production via increased thyroid hormone release during the winter months (Hassi et al 2001). Thyroid hormones show seasonal modulation in sheep, and are necessary during an interval late in the breeding season to promote seasonal reproductive suppression (Thrun et al 1997).…”

Cold Adaptations

“…After a while, we can observed a decrease in FT4 sometimes associated with a decrease in TT4 and a rise in TSH setting up the "polar syndrome of T3" [50][51][52][53][54][55][56] . But the rise in TSH is inversely related to the intensity of the light, so it is not always retrieved 51) .…”

“…After a while, we can observed a decrease in FT4 sometimes associated with a decrease in TT4 and a rise in TSH setting up the "polar syndrome of T3" [50][51][52][53][54][55][56] . But the rise in TSH is inversely related to the intensity of the light, so it is not always retrieved 51) . Last, the biological significance of these changes in the thyroid hormones is unknown 57) and the relationship between this syndrome and one of a type of cold adaptation has yet to be defined 58,59) .…”