Serum ionized calcium, as well as phosphorus and parathyroid hormone, is associated with the plasma 1,25‐dihydroxyvitamin D 3 concentration in normal postmenopausal women

Serum parathyroid hormone (PTH) and low‐normal serum phosphorus (P) concentrations have well‐known trophic effects on renal 1‐hydroxylase. A role for serum ionized calcium (Ca 2+ ) in the day‐to‐day regulation of 1,25‐dihydroxyvitamin D 3 [1,25‐(OH) 2 D 3 ] has not been identified in normal humans. The associations between serum Ca 2+ , PTH, P, and plasma 1,25‐(OH) 2 D 3 were evaluated in a cross‐sectional study of 275 healthy postmenopausal women. Partial correlations of Ca 2+ , PTH, and P (each controlled for the other two) with 1,25‐(OH) 2 D 3 were sought within quintiles of Ca 2+ At low‐normal concentrations (<1.26 mmol/liter, quintile 1) Ca 2+ was inversely correlated with 1,25‐(OH) 2 D 3 ( r p = ‐0.30, p = 0.028). At midnormal concentrations Ca 2+ was not significantly correlated with 1,25‐(OH) 2 D 3 . At high‐normal concentrations >1.32 mmol/liter, quintile 5) Ca 2+ attenuated the positive associations of both PTH and low‐normal P with 1,25‐(OH) 2 D 3 . In quintile 5 Ca 2+ , PTH, and P together accounted for none of the variability in 1,25‐(OH) 2 D 3 ( R 2 = 0.03, p = 0.671). Women with Ca 2+ below 1.32 mmol/liter were next examined by quintile of P. As expected, at low‐normal concentrations (<1.03 mmol/liter, quintile 1) P was significantly correlated with 1,25‐(OH) 2 D 3 ( r p = ‐0.32, p = 0.047). The association between PTH and 1,25‐(OH) 2 D 3 was statistically significant only at midnormal concentrations of P ( r p = 0.52, p = 0.001, quintile 3). We conclude that Ca 2+ , along with PTH and P, is associated with the plasma concentration of 1,25‐(OH) 2 D 3 in normal postmenopausal women.