Abstract

The control of calcium metabolism is dominated by the D vitamins which have different origins and relative activities. Calcitriol (1, 25-dihydroxycholecalciferol) probably mediates most of the actions of vitamin D, though possibly 25-hydroxycholecalciferol may be important in maintaining normal mineralisation.¹ Calcitriol is normally synthesised by two hydroxylation steps (see figure), first in the liver and then in the kidney, from the vitamin D₃ (cholecalciferol) produced in the skin or taken in the diet. Calcitriol (Rocaltrol - Roche) is now available on prescription, as is lα-OH vitamin D₃ (alfacalcidol - One-Alpha - Leo).² This article discusses their use.