Decreased 1,25-dihydroxycholecalciferol and increased 25-hydroxy- and 24,25-dihydroxycholecalciferol in tissues of rats treated with thyroxine.

The effect of thyroxine on the metabolism of vitamin D was investigated in rats. Vitamin D depleted rats were repleted by injections of radiolabelled cholecalciferol or 25-hydroxycholecalciferol (25OHD3). After 3 weeks, a state of hyperthyroidism was induced by daily injections of L-thyroxine for 21 days. The lipid extracts of the Plasma and tissues were analyzed by successive Sephadex LH-20 and high pressure liquid chromatography. The concentrations of 25OHD3 and of 24,25-dihydroxycholecalciferol (24,25 (OH)2D3) were significantly higher and those of cholecalciferol and of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) were significantly lower in the plasma and tissues of animals treated with thyroxine than in controls. The present study suggests that alterations in the metabolism of vitamin D may be involved in the disturbances of calcium metabolism observed in hyperthyroidism.