Regulation and adaptation of endocrine axes at high altitude.

As a model of extreme conditions, eight healthy women, part of a 40-member Nepal mountain-climbing expedition, were monitored for dynamic endocrine adaptations. Endocrine measurements were made at frequent intervals over a 6-10 hours period at four altitudes: 450 m, 4800 m (Base Camp), 6050 m and again at 4800 m (on descent) after an acclimatization period (4800 mA). Quantified hormones were growth hormone (GH), prolactin (PROL), Cortisol (Cort), Thyroid-stimulating hormone (TSH), and free thyroxine. These hormones are important to the anabolic/catabolic balance of the body, and are vital to growth, homeostasis, hypothalamic inhibition, regulation of stress and metabolism. A key secondary question was the degree to which acclimatization can stabilize hormonal disruption. Based upon statistical false discovery rates, the present analyses unveil marked adaptive changes in the thyroid axis at the level of pulsatile secretion of the pituitary hormone TSH and its downstream product, free thyroxine; strong effects upon the mass of GH, TSH, Cortisol and PROL secretion per burst; and prominent pulsatile frequency disruption and recovery for PROL and cortisol. Since pulsatility changes reflect de facto perturbations in hypothalamo-pituitary control mechanisms, the present data introduce the concept of both frequency and amplitude-dependent adaptive control of brain-pituitary neuroendocrine signals under conditions of extreme altitude exertion and exposure.