Effects of high altitude and cold exposure on resting thyroid hormone concentrations

This study examined the effect of high altitude and cold exposure on thyroid hormone status during a mountaineering expedition. There were 15 males participating in an expedition to climb Mt. McKinley, AK. Resting blood samples were collected and analyzed for thyroid-stimulating hormone (TSH), total (T)-free (f) thyroxine (T4), triiodothyronine (T3), reverse T3 (rT3), and cortisol. Measurements were made on three occasions: baseline in the continental U.S. one week pre-expedition (PRE-I), baseline in Alaska immediately pre-expedition (PRE-II), and immediately upon descent post-expedition (POST). Statistical analysis indicated that no significant trial PRE-I vs. PRE-II (p > 0.05) differences occurred. Significant (p < 0.05) reduction, however, occurred from PRE-II to POST expedition in TSH, TT3, and fT3. Also rT3 and cortisol increased significantly PRE-II to POST while TT4 and fT4 were unchanged. The increase (delta = POST minus PRE-II) in cortisol was negatively correlated with the decrease (delta) in TSH (r = -0.52, p = 0.05) and TT3 (r = -0.49, p < 0.06). Moreover, the reduction in TT3 was inversely correlated with the rT3 increase (delta comparison, r = -0.66, p < 0.01). The findings demonstrate that the resting concentrations of thyroid hormones are disrupted by a mountaineering expedition, specifically an environmental stress-related "low T3 condition" seems to develop. These changes would seem to be related to an impaired peripheral conversion of T4 to T3, possibly brought about by elevations in the circulating cortisol levels.