Influence of vitamin D on parathyroid function in the elderly.

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Abstract
The effect of age and vitamin D status on parathyroid function was studied in 129 healthy subjects between 20 and 89 yr old, with normal serum creatinine (less than 0.11 mmol/L), and living in Cordoba, Spain. Serum calcium and phosphorus as well as 25-hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D [1,25-(OH)2D] decreased, whereas serum alkaline phosphatase increased, with age. Serum PTH also increased significantly with age when measured with either a carboxyl-terminal (cPTH) or an intact [PTH(1-84)] assay. The increase in cPTH, however, exceeded largely the increase in PTH(1-84) (+120% and +30% in subjects above 80 yr vs. young adults, respectively). Serum PTH(1-84) was negatively correlated with serum (ionized) calcium, 25OHD, and insulin-like growth factor I (IGF-I) but not with serum 1,25-(OH)2D. Serum 1,25-(OH)2D decreased with age and was highly correlated with serum 25OHD, cPTH, and IGF-I. In multiple regression analysis 50-60% of the variation of total and free 1,25-(OH)2D could be explained by serum 25OHD, PTH(1-84), and especially IGF-I, suggesting a possible role of decreasing GH and IGF-I concentrations in the mineral homeostasis of the elderly. Calcium infusion (1.5 mg/kg body weight over 10 min) decreased serum PTH(1-84) to below normal concentrations in young adults (nadir 14% of basal concentration), whereas the nadir in elderly subjects with secondary hyperparathyroidism was only 32% of basal concentration. The relative decrease was, however, identical in both age groups when simultaneous changes in ionized calcium were taken into account. Basal serum PTH(1-84) in selected elderly subjects (50 +/- 10 ng/L or 5 +/- 1 pmol/L, n = 10) decreased significantly (2.7 +/- 0.9 pmol/L, P less than 0.01) after 3 iv injections of 1,25-(OH)2D during 1 week without changes in serum (ionized) calcium. The PTH suppressibility after calcium infusion did not further improve.

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