DIRECT STIMULATION OF BONE-RESORPTION BY THYROID-HORMONES

Although hypercalcemia, osteoporosis and increased bone turnover are associated with thyrotoxicosis, no direct effects of thyroid hormones on bone metabolism have been reported previously in organ culture. Prolonged treatment with thyroxine (T4) or triiodothyronine (T3) can directly increase bone resorption in cultured fetal rat long bones as measured by the release of previously incorporated 45Ca. T4 and T3 at 1 .mu.M-10 nM increased 45Ca release by 10-60% of total bone 45Ca during 5 days of culture. The medium contained 4 mg/ml of bovine serum albumin to which 90% of T4 and T3 were bound, so that free concentrations were less than 0.1 .mu.M. The response to T4 and T3 was inhibited by cortisol (1 .mu.M) and calcitonin (100 mU[m units]/ml). Indomethacin did not inhibit T4 response, suggesting that T4 stimulation of bone resorption was not mediated by increased prostaglandin synthesis by the cultured bone. Matrix resorption was demonstrated by a decrease in extracted dry weight and hydroxyproline concentration of treated bones and by histologic examination which showed increased osteoclast activity. The effects of thyroid hormones were not only slower than those of other potent stimulators of osteoclastic bone resorption (parathyroid hormone, vitamin D metabolites, osteoclast activating factor and prostaglandins), but the maximum response was not as great. T4 and T3 can directly stimulate bone resorption in vitro at concentrations approaching those which occur in thyrotoxicosis. This effect may explain the disturbances of Ca metabolism seen in hyperthyroidism.