Parathyroid hormone in relation to various vitamin D metabolites in adult females

Vitamin D binding protein (DBP) and albumin are the important determinants of circulatory 25(OH)D in adults. Physiological function of vitamin D is particularly regulated by DBPs. Serum parathyroid hormone (PTH) is considered as the biological activity reader of circulating 25(OH)D. We therefore examined the relationships between serum total, free, and bioavailable 25(OH)D versus PTH in apparently healthy Saudi female adults. A total of 350 apparently healthy Saudi female adults ([Mean ± standard deviation] age [years] 52.9 ± 9.2; body mass index [kg/m(2)] 32.9 ± 5.4) were included in this observational study. Anthropometrics was measured as well as fasting glucose, lipid profile, calcium and phosphorous using routine methods. Serum 25(OH)D was measured using an electrochemiluminescence immunoassay. Serum DBP was determined by ELISA. Free and bioavailable 25(OH)D were calculated by comparing concentrations of total 25(OH)D, DBP, and albumin. Data revealed that circulating total 25(OH)D had weak but significant inverse association with DBP (R = −0.24; P < .01), and strong inverse associations with free 25(OH)D (R = −0.87; P < .001), albumin-bound 25(OH)D (R = −0.88; P < .001), and bioavailable 25(OH)D (R = −0.89; p < 0.001). None of the vitamin D metabolites, including 25(OH)D, correlated with serum PTH. Various metabolites of 25(OH)D are not correlated with serum PTH in Saudi adult females. Bioavailable, albumin-bound and free 25(OH)D cannot be surrogate measures for vitamin D status, at least in this population.