Treatment with active vitamin D (alphacalcidol) in patients with mild primary hyperparathyroidism.

The parathyroid gland possesses receptors for 1,25-dihydroxyvitamin D3, the active metabolite of the vitamin D system, and in vitro experiments have shown that 1,25-dihydroxyvitamin D3 can inhibit the secretion of PTH. In this study 31 subjects who had displayed persistent mild hypercalcemia for 14 years and presumably had mild primary hyperparathyroidism (HPT) were challenged with 1.0 microgram alphacalcidol (1 alpha-(OH)-vitamin D3) over 6 months in a double-blind, placebo-controlled study. Before initiation of therapy, the hyperparathyroid subjects showed lower serum levels of 1,25-dihydroxyvitamin D in relation to PTH or calcium when compared with age- and sex-matched controls. Treatment induced a slight rise in serum calcium (0.05 mmol/l), but no significant decrease of the PTH levels. Eighteen of the subjects thereafter entered an open study with a higher dose of alphacalcidol (2.0 micrograms) over 1 year. Although this high dose induced a marked rise in serum calcium (0.17 mmol/l), there was only a transient reduction of the PTH levels. Thus, during long-term condition there was an escape from the suppressive action of the elevated calcium concentrations and no evidence of a specific inhibition of PTH secretion by a small oral dose of active vitamin D.