1. The elimination half-time of 25-hydroxyvitamin D in plasma was estimated after intravenous injection of the radioactively labelled metabolite in seven patients with primary hyperparathyroidism before and after excision of a parathyroid adenoma.
2. The elimination half-time of 25-hydroxyvitamin D was significantly shortened in primary hyperparathyroidism and reverted towards normal after parathyroidectomy.
3. The increased metabolic clearance of 25-hydroxyvitamin D in primary hyperparathyroidism was accounted for by an increased excretion of vitamin D-derived inactivation products in the faeces.
4. Enhanced hepatic inactivation of 25-hydroxyvitamin D may be important in the development of vitamin D deficiency in primary hyperparathyroidism.