Vitamin D status: effects on parathyroid hormone and 1,25-dihydroxyvitaminD in postmenopausal women

Background: Low serum 25-hydroxyvitamin D [25(OH)D] concentrations are comm only found in the elderly and are associated with hip fracture. Treatment w ith Vitamin D and calcium can reduce the risk of fracture. The relation bet ween the rise in parathyroid hormone (PTH) with age and the decrease in 25( OH)D is not clear. Neither is there any consensus on the serum concentratio n of 25(OH)D required for bone health. Objective: Our objective was to study the relations between serum PTH, seru m vitamin D metabolites, and other calcium-related variables in postmenopau sal women. Design: This was a cross-sectional study of 496 postmenopausal women withou t vertebral fractures attending our menopausal osteoporosis clinics. Results: PTH was significantly positively related to age and serum 1,25-dih ydroxyvitamin D [1,25(OH)(2)D] and inversely related to 25(OH)D and plasma ionized calcium. There was a step-like increase in PTH as serum 25(OH)D fel l below 40 nmol/L. In women with 25(OH)D concentrations >40 nmol/L, 1,25(OH )(2)D was positively related to 25(OH)D; in women with 25(OH)D concentratio ns less than or equal to 40 nmol/L, the relation was the inverse. In women with 25(OH)D concentrations less than or equal to 40 nmoI/L, 1,25(OH)(2)D w as most closely related to PTH; in women with 25(OH)D concentrations >40 nm ol/L, 1,25(OH)(2)D was most closely (inversely) related to plasma creatinin e. Therefore, with serum 25(OH)D concentrations increasingly <40 nmol/L, se rum 1,25(OH)(2)D becomes critically dependent on rising concentrations of P TH. Conclusion: The data suggest that aging women should maintain 25(OH)D conce ntrations >40 nmol/L (which is the lower limit of our normal range for heal thy young subjects) for optimal bone health.