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.
|Journal||American Journal of Physiology - Endocrinology and Metabolism|
|State||Published - 1988|