Calcium and vitamin D metabolism in acromegaly

Acromegalic subjects were found to have elevated serum levels of both 1,25-dihydroxyvitamin D (1,25-(OH)2D), 67 +/- 22 (SD) pg/ml) and 24.25-dihydroxyvitamin D (24.25-(OH)2D), (6.9 +/- 1.5 (SD) ng/ml). The serum concentration of 1,25-(OH)2D correlated positively (P less than 0.02, R = 0.56) to the 24 h urinary excretion of growth hormone, but not to the serum levels of parathyroid hormone, prolactin, thyroid hormones or the urinary excretion of free cortisol. Fourteen patients were treated with bromocriptine at doses from 15-45 mg/day for a period of about 6 months. This was accompanied by a significant decrease in the urinary excretion of growth hormone and calcium and in the serum concentrations of 1,25-(OH)2D and 24.25-(OH)2D. A relationship was demonstrated between the decrease in urinary calcium excretion and the decrease in serum 1,25-(OH)2D (P less than 0.02, R = 0.64). It is concluded that the serum concentration of 1,25-(OH)2D is elevated in acromegaly, perhaps as a consequence of a direct action of growth hormone on the renal 1 alpha-hydroxylase activity.