The effect of growth hormone on parathyroid function in rats

The effect of excessive growth hormone (GH) on parathyroid function in rats was evaluated in order to determine whether the GH-induced increase in serum calcium (Ca) may be mediated via stimulation of parathyroid hormone (PTH) secretion. Rats receiving injections of bovine GH 2 mg daily for 4 weeks showed a progressive significant increase in both serum Ca and immunoreactive PTH (iPTH) after the second week. Rats receiving daily injections of rat GH, 0.25 mg daily for 3 weeks, followed by 0.5 mg daily for 3 additional weeks, showed a significant increase in serum iPTH during administration of the higher dose, and also a significantly increased parathyroid gland weight and PTH content. The concomitant increase in serum Ca and iPTH suggest that GH stimulates the parathyroid glands to increase PTH secretion, which in turn causes an increase in serum Ca. The parathyroid response increases with the increase in dose and duration of GH stimulus. The data suggest that the hypercalcemia often observed in acromegaly may be due, at least in part, to GH-induced excessive secretory activity of the parathyroid glands.