Decreased free fraction of thyroid hormones after prolonged Antarctic residence

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."