AN INVESTIGATION OF THE BIOLOGICAL ACTIVITY OF PARATHYROID HORMONE IN PSEUDOHYPOPARATHYROIDISM: COMPARISON WITH VITAMIN D DEFICIENCY

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Abstract
Circulating parathyroid hormone [PTH] was studied in patients with pseudohypoparathyroidism and compared with that in normal subjects and paients with hypocalcemia 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 PTH 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 hypocalcemia was similar in these 2 groups of patients. By contrast, the concentration of bioactive PTH was low (< 1.6 pg/ml) in 4 patients with untreated postsurgical or idiopathic hypoparathyroidism. Restoration of normocalcemia reduced the concentrations of bioactive PTH in both pseudohypoparathyroidism and vitamin D deficiency. In this respect the parathyroid glands in both conditions appeared to respond to the circulating Ca concentration. Immunoreactive PTH was also raised in patients with untreated pseudohypoparathyroidism and vitamin D deficiency, but restoration of normocalcemia did not always reduce immunoreactive PTH to normal in these patients. There can be dissociation between bioactivity and immunoreactivity even when the PTH is measured in an amino-terminal specific assay.