indication

Used for the prevention and treatment of rickets or osteomalacia, and to manage hypocalcemia associated with hypoparathyroidism or pseudohypoparathyroidism. Also used for the treatment of vitamin D dependent rickets, rickets or osteomalacia secondary to long-term high dose anticonvulsant therapy, early renal osteodystrophy, osteoporosis (in conjunction with calcium), and hypophosphatemia associated with Fanconi syndrome (with treatment of acidosis).

pharmacology

Dihydrotachysterol is hydroxylated in the liver to 25-hydroxydihydrotachysterol, which is the major circulating active form of the drug. It does not undergo further hydroxylation by the kidney and therefore is the analogue of 1, 25-dihydroxyvitamin D. Dihydrotachysterol is effective in the elevation of serum calcium by stimulating intestinal calcium absorption and mobilizing bone calcium in the absence of parathyroid hormone and of functioning renal tissue. Dihydrotachysterol also increases renal phosphate excretion.

mechanism of action

Once hydroxylated to 25-hydroxydihydrotachysterol, the modified drug binds to the vitamin D receptor. The bound form of the vitamin D receptor serves as a transcriptional regulator of bone matrix proteins, inducing the expression of osteocalcin and suppressing synthesis of type I collagen. Vitamin D (when bound to the vitamin D receptor)stimulates the expression of a number of proteins involved in transporting calcium from the lumen of the intestine, across the epithelial cells and into blood. This stimulates intestinal calcium absorption and increases renal phosphate excretion. These are functions that are normally carried out by the parathyroid hormone.

toxicity

Toxicity associated with dihydrotachysterol is similar to that seen with large doses of vitamin D.