Vitamin D and the parathyroid.

Vitamin D's biologically active metabolite, 1,25(OH)2D3, has important effects upon the parathyroid cell that are relevant to both the physiology of mineral metabolism and the regulation of the secondary hyperparathyroidism of chronic renal failure. 1,25(OH)2D3 markedly decreases parathyroid hormone (PTH) gene transcription and thus PTH synthesis and secretion. It also acts to decrease parathyroid cell proliferation. Nonhypercalemic analogs of 1,25(OH)2D3 are being developed that may have a wider therapeutic window than 1,25(OH)2D3 itself. In the situations of chronic hypocalcemia and hypophosphatemia, there are interesting interrelationships between 1,25(OH)2D3 and the post-transcriptional regulation of the PTH gene. In nodular secondary hyperparathyroidism, there is down-regulation of the vitamin D receptor in the parathyroid. Different vitamin D receptor genotypes may be associated with higher levels of serum PTH and a predisposition to autonomous hyperplasia.