Hypophyseal Hormonal Response To Hypothalamic Factors In Hypoxia

Exposure to high altitude induces various physiological changes, including activation or inhibition of hormonal systems. In response to activation processes, a desensitization of several pathways has been described, especially in the adrenergic system. In the present study, we aimed to assess if the hypophyseal hormones are also subjected to a hypoxia-induced decrease in their response to hypothalamic factors. Basal levels of hormones and the responses of TSH, thyroid hormones, prolactin, sex hormones and growth hormone to the injection of TRH, GnRH and GHRH were studied in 8 men in normoxia and on prolonged exposure (3–4 days) to an altitude of 4,350 m. Thyroid hormones were elevated at altitude (+16 to +21%), while TSH levels were unchanged, FSH and prolactin decreased while LH was unchanged. Norepinephrine and cortisol were elevated while no change was observed in epinephrine, dopamine, GH, IGF-1 and IGF-BP3. The mean response to hypothalamic factors was similar in both altitudes for all studied hormones, although total T4 was lower in hypoxia during 45 to 60 minutes after injection. The effect of hypoxia on the hypophyseal response to hypothalamic factors was similar among subjects, except for the GH response to GHRH administration. We conclude that prolonged exposure to altitude hypoxia induces contrasted changes in hormonal levels. The hypophyseal response to hypothalamic factors does not appear to be blunted.