Reduced calcitonin reserve in young hypogonadic osteoporotic men

Calcitonin is a potent inhibitor of bone resorption and in both sexes, plasma levels progressively decrease with age: therefore, a relative deficiency of calcitonin may be involved in the pathogenesis of osteoporosis in the elderly. Calcitonin plasma levels of young hypogonadic men with osteoporosis are significantly lower than controls: the hypothesis that the decreased calcitonin plasma levels in the elderly are due to a reduced secretory capacity of the "C" cells of the thyroid gland, related to age, does not explain the low calcitonin plasma levels found in young hypogonadic osteoporotic men. Our hypothesis is that gonadal steroid deficiency may participate in the mechanisms regulating calcitonin secretion. Therefore, we studied ten males affected by hypogonadotropic hypogonadism and ten normal men, of comparable age, as controls: we measured plasma levels of testosterone, 17 beta estradiol, androstenedione and calcitonin, and the response of calcitonin to an i.v. bolus of pentagastrin, a well known "C" cells stimulatory drug. Testosterone and calcitonin plasma levels and the response of calcitonin to pentagastrin were also evaluated after 6 months of replacement therapy with testosterone. Basal levels of testosterone, 17 beta estradiol, androstenedione and calcitonin, and the response of calcitonin to pentagastrin, are significantly lower in our patients than in controls, demonstrating that hypogonadotropic hypogonadic subjects have a lower secretory reserve of calcitonin. After testosterone therapy the basal calcitonin plasma levels and its response to pentagastrin stimulus did not differ from controls, suggesting that gonadal steroids influence the calcitonin secretion and reserve. Our data cannot clarify whether osteoporosis of hypogonadotropic hypogonadic patients is related to androgen or estrogen deficiency; however, they suggest that the mechanisms by which gonadal steroid influence bone metabolism may involve calcitonin secretion.