Effect of Thyrotoxicosis and Its Treatment on Bone Mineral and Muscle Mass*

Thyrotoxicosis is a known cause of loss of bone and muscle mass. Bone mineral mass was studied by partial body in vivo neutron activation analysis, in which stable 48Ca is converted to radioactive 49Ca and measured in a whole body counter. A calcium bone index (CaBI) has been established that normalizes the Ca measurements for body size. Muscle mass was measured by counting the natural radioactive 40K in a whole body counter. A potassium index (Ki) was obtained, which, as with the CaBI, normalized potassium for body size. Forty-one hyperthyroid patients were studied, with sequential measurements on 13 subjects for up to 50 months after they became euthyroid. For the 37 patients who were under 55 yr of age, the mean CaBI was not significantly differentfrom that of the controls (0.95 ± 0.15 compared to 1.00 ± 0.12). Of the total 41 patients, however, 9 had CaBI values below the normal range (2 SD below the normal mean), reflectingbone loss. The mean Ki [ 0.88 ± 0.08 (1 SD)] in patients under 55 yr of age was below (P < 0.01) the normal mean [1.00 ± .10 (1 SD)], reflecting muscle wasting inthe group as a whole. After successful therapy, the 13 patients, on the average, showed significant increases in both CaBI (from 0.87 ± 0.15 to 0.98 ± 0.09) and Ki (from 0.89 ± 0.06 to 1.02 ± 0.09), demonstrating reversible loss in both bone mineral content and muscle mass in thyrotoxicosis.