Serum ionized calcium, as well as phosphorus and parathyroid hormone, is associated with the plasma 1,25-dihydroxyvitamin D3 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 (Ca2+) in the day-to-day regulation of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] has not been identified in normal humans. The associations between serum Ca2+, PTH, P, and plasma 1,25-(OH)2D3 were evaluated in a cross-sectional study of 275 healthy postmenopausal women. Partial correlations of Ca2+, PTH, and P (each controlled for the other two) with 1,25-(OH)2D3 were sought within quintiles of Ca2+. At low-normal concentrations (less than 1.26 mmol/liter, quintile 1) Ca2+ was inversely correlated with 1,25-(OH)2D3 (rp = -0.30, p = 0.028). At mid-normal concentrations Ca2+ was not significantly correlated with 1,25-(OH)2D3. At high-normal concentrations (greater than 1.32 mmol/liter, quintile 5) Ca2+ attenuated the positive associations of both PTH and low-normal P with 1,25-(OH)2D3. In quintile 5 Ca2+, PTH, and P together accounted for none of the variability in 1,25-(OH)2D3 (R2 = 0.03, p = 0.671). Women with Ca2+ below 1.32 mmol/liter were next examined by quintile of P. As expected, at low-normal concentrations (less than 1.03 mmol/liter, quintile 1) P was significantly correlated with 1,25-(OH)2D3 (rp = -0.32, p = 0.047). The association between PTH and 1,25-(OH)2D3 was statistically significant only at mid-normal concentrations of P (rp = 0.52, p = 0.001, quintile 3). We conclude that Ca2+, along with PTH and P, is associated with the plasma concentration of 1,25-(OH)2D3 in normal postmenopausal women.