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Changes in Serum Triiodothyronine (T₃) Kinetics after Prolonged Antarctic Residence: The Polar T₃ Syndrome^*

Department of Environmental Medicine, Naval Medical Research Institute (H.L.R.) Bethesda, Maryland 20814-5055
the Departments of Nuclear Medicine (E.D.S.) and Medicine (H.L.R., K.M.M.S., R.D., K.D.B., J.T.O.) and the Endocrine-Metabolic Service (H.L.R., K.M.M.S., R.D., J.T.O.), National Naval Medical Center, and the Departments of Medicine (H.L.R., KM.M.S., R.D., J.T.O.) and Radiology and Nuclear Medicine (E.D.S.), Uniformed Services University of the Health Sciences Bethesda, Maryland 20814-5011
the Department of Medicine, Endocrine and Metabolic Service, Kyle Metabolic Unit, Department of Clinical Investigation, Walter Reed Army Medical Center (K.D.B.) Washington, D.C. 20307-5100

Address all correspondence and requests for reprints to: H. L. Reed, M.D., Mail Stop 11, Environmental Medicine Department, Naval Medical Research Institute, Bethesda, Maryland 20814-5055.

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.

^* The opinions expressed herein are those of the authors and are not to be construed as reflecting the views of the Department of the Navy, the Department of the Army, the Department of Defense, or the NSF. This work was supported in part by U.S. Navy Research and Development Command work unit 63706N M0095.004.1008 and NSF Grant S-035.

Received April 14, 1989.

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