Changes in serum triiodothyronine (T₃) kinetics after prolonged antarctic residence: The polar T₃ syndrome

Humans who live in Antarctica for greater than 5 continuous months demonstrate alterations in the hypothalamic-pituitary-thyroid axis. These changes are characterized by 1) increased pituitary release of TSH in response to iv TRH, 2) increased serum clearance of orally administered T₃, and 3) normal serum total, free T₄, and unstimulated TSH levels. To clarify the mechanism responsible for these findings, serum kinetic studies of ¹²⁵I-labeled T₄ and T₃ were carried out in a group of normal men, first in California, then after 20 and 42 weeks of continuous Antarctic residence. The kinetic parameters were calculated by noncompartmental analysis. The mean T₄ residence time (MRT) was not different before and after 42 weeks (5.54 ± 0.50 and 5.08 ± 0.43 days). The total T₄ volume of distribution (TVd) tended to fall over the same period (4.30 ± 0.12, 3.56 ± 0.27 L/m²), but was not significantly different (P = 0.075). In contrast to T₄, there was an increase from control values for the T₃ MRT from 0.83 ± 0.03 to 1.10 ± 0.03 days (P < 0.002) and a more than doubling of the T₃ TVd from 15.55 ± 0.52 to 47.24 ± 5.09 L/m² (P < 0.002) after 42 wk of Antarctic residence. Energy intake increased approximately 40% throughout the study without a change in body weight. The changes in T₃ kinetic parameters may be accounted for by increased extravascular tissue binding. The marked increase in T₃ TVd and the small increase in MRT are associated with increased T₃ production and clearance and only minor changes in T₄ kinetics. This is the first description of a mechanism for the change in thyroid hormone economy occurring with extended residence in Antarctica.