Pituitary and peripheral hormone responses to T₃ administration during Antarctic residence

Very little is known regarding hormonal adaptation in human subjects who are exposed to the extremes of temperature and light that are found in polar latitudes. We have previously reported a 50% elevation in the serum thyrotropin (TSH) response to thyrotropin-releasing hormone (TRH), a fall in serum total triiodothyronine (T₃) and free T₃ (fT₃), and no change in serum total thyroxine (T₄) or free T₄ (fT₄) after 42 wk of Antarctic cold exposure. To differentiate between central and peripheral mechanisms that may lead to these changes, we report the effect of sequentially increasing oral doses of T₃ (Cytomel) on serum T₃ and fT₃ levels and on the resultant attenuation of the TSH response to TRH in nine men before, during, and after 42 wk residence in Antarctica. Serum T₃ values basally and following the administration of 25, 50, and 75 μg/day of T₃ were lower after 42 wk of cold exposure (151 ± 4, 160 ± 8, 189 ± 10, and 222 ± 14 ng/dl, respectively, compared with control values of 160 ± 7, 178 ± 7, 202 ± 9, and 251 ± 19 ng/dl, respectively, P < 0.05). Likewise, the fT₃ values measured after these three increasing T₃ doses were also lower after 42 wk of cold exposure. The pituitary response to TRH was attenuated by each T₃ regimen (48 ± 6, 68 ± 4, and 77 ± 4% decreases in the control period), and this suppression was not different after 20 and 42 wk of Antarctic residence. Serum T₄ and fT₄ values were similar throughout the study. We conclude that the pituitary sensitivity to T₃ was unchanged during the study and that changes in TSH responsiveness and serum T₃ levels were likely due to changes in peripheral T₃ metabolism.