Possible Limited Bone Loss with Suppressive Thyroxine Therapy Is Unlikely to Have Clinical Relevance

To determine the effect of suppressive doses of thyroxine (T4) on bone mass, we studied 50 women on suppressive doses of T4 for 3-27 years (mean of 11 years). Twenty-five had nontoxic goiter and 25 had well-differentiated thyroid carcinoma. Fifty controls were matched for age, menopausal status, and body mass index. Bone mineral density (BMD) was measured in the lumbar spine (LS), femoral neck (FN), trunk (TK), and extremities (EXT) by dualenergy X-ray absorptiometry (DXA). In addition, the trunk area was measured by neutron activation analysis and recorded as a calcium bone index (CaBI). Twenty-one patients were restudied with DXA measurements at a mean of 1.5 ± 0.5 (1 SD) years. The total population of 50 patients showed no difference in bone mass from controls. In patients with nontoxic goiter, there was no evidence of any loss in bone mass. Cancer patients showed insignificant reductions of 2-5% in BMD of LS, FN, and TK and a significant 5% reduction in BMD of EXT, compared to controls, and a 12% reduction in CaBI compared to goiter patients. Cancer patients had a slightly higher (p < 0.001) mean daily dose of T₄ than goiter patients (0.23 vs 0.15 mg/day) but had a similar degree of TSH suppression. BMD and CaBI values did not correlate with free T4 index, with the daily T4 dose, accumulative dose, or with duration of T4 therapy. There were no significant changes in bone mass in either goiter or cancer patients restudied after a mean of 1.5 years. These findings suggest that suppressive T4 therapy does not affect bone mass in patients with nontoxic goiter. In patients with thyroid cancer, limited bone loss was detected. No osteoporotic fractures were documented in any of the patients. The minimal amount of bone loss, which was barely significant, is unlikely to have clinical relevance.