Comparative response to parathyroid hormone in hyperthyroidism and hypothyroidism.

The effects of exogenous parathyroid hormone, administered for 3 days, were compared in six hyperthyroid and six hypothyroid subjects. Maximum increments were much greater in hyperthyroid than in hypothyroid subjects for serum calcium (3.5 mg/100 ml versus 1.6 mg/100 ml), urine calcium (476 mg versus 79 mg), urine hydroxyproline (56 mg versus 11 mg), and urine phosphorus (671 mg versus 192 mg). Maximum decrease in serum phosphorus (minus0.9 mg/100 ml versus minus 0.1 mg/100 ml) was also greater in hyperthyroid subjects. Serum parathyroid hormone immunoreactivity was significantly higher in hypothyroid subjects (0.48 ng/ml) that either normals (0.21 ng/ml) or hyperthyroid subjects (0.19 ng/ml). The data support the concept that excess thyroid hormone sensitizes and deficient thyroid hormone blunts the responsiveness of bone to parathyroid hormone. This may lead to a state of hypoparathyroidism in hyperthyroidism and hyperparathyroidism in hypothyroidism.