The effects of the menopause on calcitriol and parathyroid hormone: responses to a low dietary calcium stress test.

Calcium, calcitriol and PTH levels were studied in 11 premenopausal women, aged 41 +/- 3 yr (mean +/- SD), and 11 postmenopausal women, aged 42 +/- 3 yr, at baseline and in 9 women from each group during a 4-day low calcium diet. Serum dialyzable calcium and urinary hydroxyproline excretion were higher in the postmenopausal women at baseline and throughout the low calcium diet. Baseline calcitriol levels were significantly lower in the postmenopausal women, but these were associated with lower vitamin D-binding protein levels; the calculated free calcitriol index was not different between the two groups. After the low calcium diet calcitriol levels rose in both groups to similar levels. PTH levels were not different in the two groups at baseline and rose to the same level on the low calcium diet. These data indicate that estrogen deficiency at the menopause is not associated with deficient calcitriol or PTH reserve, although basal calcitriol levels may be reduced secondarily due to lower vitamin D-binding protein levels or relative hypercalcaemia.