Calcitriol corrects deficient calcitonin secretion in the vitamin D-deficient elderly.

The thyroid calcitonin-producing C cells possess vitamin D receptors and synthesize the vitamin D-dependent calbindin D28K. The present study evaluates the possible direct or indirect influence of vitamin D on calcitonin secretion in the elderly. Serum calcitonin was measured before and after a short calcium infusion (1.5 mg/kg over 10 minutes) in nine normal young adults (30 +/- 4 years, mean +/- SEM) and eight elderly subjects (78 +/- 4 years). The test was repeated 48 h after the last of three intravenous injections of calcitriol (2 micrograms) given every other day. Basal serum calcium did not change, but basal calcitonin of the elderly increased from 7 +/- 1 to 10 +/- 1 pg/ml (p < 0.06), similar to basal values in young adults (11 +/- 1 pg/ml). The increase in calcitonin after calcium infusion increased from 8 +/- 1 to 14 +/- 1 pg/ml (p < 0.001) after calcitriol treatment and approached the increase in young adults (18 +/- 3 pg/ml). These data demonstrate that calcitriol can improve and nearly normalize the impaired calcitonin secretion of the mildly vitamin D-deficient elderly subjects without changes in serum calcium, whereas the inverse situation is observed for parathyroid hormone.