Absence of effect of 24,25-dihydroxyvitamin D3 in primary hyperparathyroidism

Nineteen patients with primary hyperparathyroidism were treated with 25 micrograms 24,25-dihydroxyvitamin D3 or placebo daily for 3 months according to double-blind cross-over protocol. Serum immunoreactive PTH, total and ionized calcium, urinary calcium excretion, tubular reabsorption of phosphate/glomerular filtrate, and urinary hydroxyproline excretion did not change significantly. Serum 24,25-dihydroxyvitamin D3 levels increased significantly from 1.4 +/- 2.2 (SD) nmol/liter to 38 +/- 11 nmol/liter during the treatment period. Serum 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 levels did not change. We conclude that pharmacological doses of 24,25-dihydroxyvitamin D3 have no suppressive effect on parathyroid function in primary hyperparathyroidism.