During the intestinal absorption of a meal, insulin promotes the uptake of blood glucose into skeletal muscle and other tissues. This lowers the blood glucose concentration and increases the energy reserves of glycogen, fat, and protein. During periods of fasting, insulin secretion decreases and glucagon secretion increases. The effects of epinephrine are similar to those of glucagon. Epinephrine stimulates glycogenolysis and lipolysis, and activates increased metabolism of brown fat.
Thyroxine stimulates the rate of cell respiration in almost all cells in the body.
The secretion of growth hormone is regulated by releasing and inhibiting hormones from the hypothalamus. The secretion of growth hormone is stimulated by a protein meal and by a fall in glucose, as occurs during fasting. The anabolic effects of growth hormone, including the stimulation of bone growth in childhood, are believed to be produced indirectly via polypeptides called insulin-like growth factors, or somatomedins. This serves as a reserve supply of calcium and phosphate for the blood. The formation and resorption of bone are produced by the action of osteoblasts and osteoclasts, respectively.