Decreased free fraction of thyroid hormones after prolonged Antarctic residence

Abstract

We investigated the effects of Antarctic residence (AR) on serum thyroid hormone and cardiovascular responses to a 60-min standard cold air (0 degree C) test (SCAT). Serum total thyroxine (TT4) and serum total triiodothyronine (TT3), free T4 (FT4) and T3 (FT3), thyrotropin (TSH), and percent free fraction of T4 (%FT4) and T3 (%FT3) were measured in normal men (n = 15) before and after each of three SCATs. The SCAT was first carried out in California and then repeated after 24 and 44 wk AR. Mean arterial pressure (MAP) and sublingual oral temperature (Tor) were measured before and during each SCAT. The SCAT did not alter thyroid hormones before or after AR. The %FT4 decreased from 0.0334 +/- 0.0017 to 0.0295 +/- 0.0007% (P less than 0.002) with 44 wk AR but without a significant change in TT4 or FT4 for the same period. The %FT3 also decreased from 0.2812 +/- 0.0128 to 0.2458 +/- 0.0067% (P less than 0.005) after 44 wk AR. FT3 decreased (P less than 0.003) but TT3 and TSH were unchanged with 44 wk AR. The decrease in %FT4 and %FT3 may be theoretically accounted for by a 10% increase in either the capacity or the affinity of the serum binding proteins. The SCAT in California increased MAP and did not change Tor. After 44 wk AR, the SCAT no longer increased MAP but did lower Tor. The shift in the Tor and MAP response to the SCAT is consistent with the associated occurrence of cold adaptation during AR. We describe for the first time a decrease in the free fraction of both serum T3 and T4 present with extended polar residence and independent of a SCAT, further characterizing the recently reported "polar T3 syndrome."

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

“…Consistent with previous studies (1,3,(6)(7)(8), the placebo group experienced small but significant declines in fT 4 and the seasonal nadir of TSH by January of the summer months, suggestive of the initial phases of the Polar T 3 Syndrome (6). The placebo group also experienced significant declines in the level of plasma catecholamines (E and NE) in the summer.…”

“…Additionally, a small decline in serum-free triiodothyronine (fT 3 ) and free thyroxine (fT 4 ), a doubling in both T 3 distribution volume and plasma appearance and clearance rate, as well as a small decrease in T 4 distribution volume further help define this condition (4,7). 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).…”

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

“…In an earlier study an increase in serum total T 3 during winter, and therefore a highly significant negative correlation between total T 3 and temperature, had been reported (Maes et al 1997). The negative correlation is difficult to understand, because in the majority of the studies dealing with seasonal changes in the secretion of thyroid hormones there are rather decreases in the levels of free or total T 3 during winter (Vining et al 1983, Reed et al 1986, 1990a,b, Harford et al 1993. We interpret our results so that at the end of winter the tissue uptake/metabolic degradation of T 3 gradually exceeds its production rate and leads to decreased levels of free T 3 .…”

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) .…”