Influence of thyroid hormones on the regulation of growth hormone secretion

The regulation of growth hormone (GH) secretion involves a complex neuroendocrine control system that includes the participation of several neurotransmitters and the feedback of hormonal and metabolic substrates. The final integration of these signals occurs in the hypothalamus, which controls GH secretion through two neuropeptides: GHRH, which stimulates GH secretion; and somatostatin, which inhibits it (1). The pattern of GH secretion in both rats and humans is pulsatile and sexually dimorphic (2). The role of GHRH and somatostatin in generating GH secretory pulses has been studied extensively in rats with the use of passive immunization techniques. These studies have demonstrated that GHRH regulates GH pulses and that somatostatin regulates baseline GH values. In humans, GH secretion is pulsatile in nature. The major secretory episode of GH occurs at night and is associated with rapid eye movement (REM) sleep. Additional episodes, however, are associated with exercise, stress or nutrient intake. Growth