Regulating of growth hormone sensitivity by sex steroids: implications for therapy.

Growth hormone (GH) is an important regulator of body composition, reducing body fat by stimulating fat oxidation and enhancing lean body mass by stimulating protein accretion. The emergence of differences in body composition between the sexes during puberty suggests sex steroids modulate the action of GH. Work from our laboratory have investigated the influence of estrogens and androgens on the metabolic actions of GH in human adults. The liver is an important site of physiological interaction as it is a sex steroid responsive organ and a major target of GH action. Estrogen, when administered orally impairs the GH-regulated endocrine and metabolic function of the liver via a first-pass effect. It reduces circulating IGF-I, fat oxidation and protein synthesis, contributing to a loss of lean and a gain of fat mass. These effects occur in normal and in GH-deficient women and are avoided by transdermal administration of physiological doses of estrogen. In contrast, studies in hypopituitary men indicate that testosterone enhances the metabolic effects of GH. Testosterone alone stimulates fat oxidation and protein synthesis, both of which are enhanced by GH. Studies in GH deficiency adults have consistently reported women to be less sensitive to GH than men. In summary, estrogens and androgens exert divergent effects on the action of GH. The results provide an explanation for sexual dimorphism in body composition in adults and the gender-related response to GH replacement in hypopituitary subjects. In the management of hypopituitarism, estrogens should be administered by the parenteral route in women and testosterone be replaced in men to optimize the benefits of GH replacement.