Introduction

Growth hormone (GH) is secreted by the anterior pituitary in a pulsatile fashion. Its secretion is tightly regulated by hypothalamic factors and by feedback from peripheral factors such as serum glucose and fatty acid levels. The hypothalamic input includes the reciprocal secretion of somatostatin and growth hormone releasing hormone (GHRH). Thus, a pulse of GH is mediated by suppression of tonic hypothalamic somatostatin secretion associated with an increase in GHRH secretion. This will not be further discussed but has been reviewed in detail previously (Tannenbaum & Ling, 1984; Thorner et al., 1986). As shown in Figure 1, GH is also regulated by circulating levels of somatomedin C (otherwise known as insulin-like growth factor) which is either produced locally or in the periphery. Thus, somatomedin C inhibits GH secretion at both the pituitary and hypothalamic levels by modulating somatostatin and possibly also GHRH secretion. Another important influence on GH secretion is gonadal steroids. This occurs in both the human and in animals.

Growth hormone secretion during the life cycle in the human

Levels of GH are detectable in the fetus during the mid-trimester and remain high throughout intrauterine life. Detectable GH is found in the serum of human fetuses as early as 70 days of gestation and by mid to late second trimester, values may reach 150 ng/ml. Thereafter, GH levels decline, but at the time of birth and for several weeks thereafter, the levels remain high when compared to adult values. Following delivery, GH levels fall and remain relatively low during childhood and rise again at the time of puberty.