Actions of thiazide on vitamin D metabolism: a controlled therapeutic trial in normal women early in the postmenopause.

The effect of thiazide on vitamin D metabolism in normal postmenopausal women was studied during a twelve-month placebo-controlled clinical study. Nineteen healthy women in their early menopause were randomized for treatment with bendroflumethiazide (5 mg/d) (n = 11) or placebo (n = 8) for twelve months. All participants were given a calcium supplement of 0.5 g/d throughout the study. A significant increase (p less than 0.01) in the serum concentration of 24,25-dihydroxycholecalciferol was observed in the thiazide group. Moreover, this group showed a tendency toward decreased serum 1,25-dihydroxycholecalciferol, whereas the mean serum 25-hydroxycholecalciferol was unchanged. Except for a highly significant decrease in urinary calcium in the thiazide group (P less than 0.01) all other biochemical indices of calcium metabolism were unchanged. The present data indicate that thiazide given to early postmenopausal women has a primary effect on the renal tubules followed by a secondary change in vitamin D metabolism leading to an increase in serum 24,25-dihydroxycholecalciferol.