Plasma 1,25(OH)2 vitamin D measured by radioimmunoassay and cytosol radioreceptor assay in normal subjects and patients with primary hyperparathyroidism and renal failure.

Antibodies to 1 alpha, 25-dihydroxy vitamin D3 25-hemisuccinate linked to albumin were produced and an immunoassay for 1,25-dihydroxy vitamin D developed. Plasma 1,25-dihydroxy vitamin D concentrations were compared using an immunoassay and cytosol radioreceptor assay. Both assays gave comparable results but the immunoassay was more reproducible, slightly more sensitive and had a lower detection limit. Using the immunoassay the plasma 1,25-dihydroxy vitamin D was 110.5 pmol/l (S.D. 29.4) in normal subjects; there was no difference between males and premenopausal females. It was negatively related to plasma phosphate. In renal failure and primary hyperparathyroidism plasma 1,25(OH)2D was positively related to radiocalcium absorption. Following 1 and 2 microgram of 1,25-dihydroxy vitamin D3 given orally the peak plasma concentration occurs within 12 h.