Vitamin D and the syndromes of azotaemic osteodystrophy.

Intestinal malabsorption of calcium and the development of osteomalacia in conservatively treated renal failure is explained by a quantitative deficiency of 1,25-dihydroxycholecalciferol, which also contributes to the development of hypocalcaemia. Excess of 25-hydroxycholecalciferol can substitute for this deficiency. The presence and healing of azotaemic osteomalacia is unrelated to the prevailing plasma [Ca] x [P] product. The data suggest that "vitamin D" acts directly on bone mineralisation, but the claim that this apparent effect is normally due to 25-hydroxycholecalciferol is considered unproven. Most of the phenomena of azotaemic osteodystrophy are encountered in simple vitamin D deficiency; as in that condition, deficiency of 1,25-dihydroxycholecalciferol may be of primary significance in causing secondary hyperparathyroidism in renal failure.