Calcitriol dosage in osteomalacia, hypoparathyroidism and attempted treatment of myositis ossificans progressiva.

Mineral retention was measured during 39 metabolic balance studies in 34 patients with nutritional osteomalacia or late rickets; they were divided into 5 treatment groups consisting of oral vitamin D, artificial ultra-violet irradiation, 25-hydroxycholecalciferol (calcifediol), 1 alpha-hydroxycholecalciferol (alfacalcidol) and 1 alpha, 25-hydroxycholecalciferol (calcitriol). With the 1 alpha-hydroxylated derivatives, initial dosage of 2 to 6 micrograms daily was required to achieve optimal healing rates by comparison with other responses. Mineral retention was markedly enhanced by supplementation with microcrystalline hydroxyapatite compound (MCHC); untreated X-linked hypophosphataemic rickets healed in 7 weeks on 10 micrograms alfacalcidol daily and 6 grams MCHC daily without developing hypercalcaemia. By contrast, adult-presenting hypophosphataemic osteomalacia developed early hypercalcaemia on the same treatment; additional phosphate supplementation, without changing other treatment, abolished hypercalcaemia and improved calcium retention. A long-term crossover trial of the vitamins D in 6 patients with hypoparathyroidism suggested that relative potencies were as follows (assigning to vitamin D an arbitrary potency of l): vitamin D2 (or D3) l: dihydrotachysterol (DHT) 3: calcifediol 10: alfacalcidol 750: calcitriol 1500. The two-fold superiority of calcitriol over alfacalcidol was evident. Calcifediol and vitamin D controlled plasma calcium at comparable levels of circulating 25-hydroxyvitamin D (25-OH-D), elevated 25-OH-D persisting at least 1 to 2 years after discontinuing long-term (greater than 4 years) vitamin D. In 2 patients with myositis ossificans progressiva treated with 10 to 20 micrograms calcitriol daily, hypercalcaemia was minimized by a low-calcium diet supplemented with cellulose phosphate, suggesting that bone resorption did not play a major role in vitamin D intoxication. Net mineral loss was documented in a young male patient but not in a menopausal female, suggesting that calcitriol treatment was not likely to produce post-menopausal osteoporosis.