Circulating immunoreactive intact human parathyroid hormone (PTH) was measured by a direct immunoradiometric assay (IRMA) and the results compared with a radio-immunoassay (RIA) which required extraction and concentration prior to assay. The sensitivity of the IRMA was better than that of the RIA (0.6 vs 2.0 pmol/L). In control subjects the hPTH concentrations ranged between 0.6 and 6.7 pmol/L and in patients with hypercalcaemia due to malignant diseases, sarcoidosis and hypoparathyroidism none could be detected. In patients with primary hyperparathyroidism the concentrations ranged from 5.2 to 27.0 pmol/L. In patients with renal osteodystrophy serum human PTH concentrations ranged from 7.6 to 285 and in those with chronic renal failure but without evidence of renal osteodystrophy from 0.5 to 5.2 pmol/L. The major advantages of the IRMA are its much simpler performance and its higher sensitivity which makes studies of the physiology of PTH secretion in humans possible.

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