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°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 (T(or)) 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 < 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 < 0.005) after 44 wk AR. FT₃ decreased (P < 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 T(or). After 44 wk AR, the SCAT no longer increased MAP but did lower T(or). The shift in the T(or) 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.'