An investigation of the biological activity of parathyroid hormone in pseudohypoparathyroidism: comparison with vitamin D deficiency.

Circulating parathyroid hormone was studied in patients with pseudohypoparathyroidism and compared with that in normal subjects and patients with hypocalcaemia due to postsurgical or idiopathic hypoparathyroidism or vitamin D deficiency. The cytochemical bioassay was used to measure bioactivity and an amino-terminal specific immunoradiometric assay was used to measure immunoreactivity. In normal subjects (n = 12) the concentration of bioactive parathyroid hormone was 1.1-5.9 pg/ml. It was higher in patients with newly diagnosed, untreated pseudohypoparathyroidism (n = 4, range 20-53 pg/ml) and similarly raised in patients with untreated vitamin D deficiency (n = 9, range 23-74 pg/ml). The degree of hypocalcaemia was similar in these two groups of patients. By contrast, the concentration of bioactive parathyroid hormone was low (less than 0.6 pg/ml) in four patients with untreated postsurgical or idiopathic hypoparathyroidism. Restoration of normocalcaemia reduced the concentrations of bioactive PTH in both pseudohypoparathyroidism and vitamin D deficiency. Thus, in this respect the parathyroid glands in both conditions appeared to respond to the circulating calcium concentration. Immunoreactive PTH was also raised in patients with untreated pseudo-hypoparathyroidism and vitamin D deficiency, but restoration of normocalcaemia did not always reduce immunoreactive PTH to normal in these patients. Thus, there can be dissociation between bioactivity and immunoreactivity even when the PTH is measured in an amino-terminal specific assay.