Bone mineral density and thyroid hormone therapy.

The purpose of this study was to evaluate prospectively the evolution of femoral and vertebral bone mineral density (BMD) in hypothyroid subjects treated with replacement doses (mean +/- SD dose of L-thyroxine = 135 +/- 32 micrograms/day) as compared to an untreated group. Vertebral bone density was also measured in other patients who had been treated for at least 2 years with either suppressive (mean dose = 154 +/- 36 micrograms/day, n = 28) or replacement doses (mean dose = 104 +/- 52 micrograms/day, n = 21) according to the thyrotrophin response (TSH) to the thyrotrophin releasing hormone (TRH) administration. In primary hypothyroid patients, a mean decrease of 5.4% (P less than 0.01) for vertebral BMD, 7.3% (P less than 0.01) for trochanter and 7% (P less than 0.001) for femoral neck was observed after 1 year of treatment. This loss was unrelated either to age or to menopausal status (ANOVA). A clinical and hormonal state of euthyroidism was reached since the 3rd month of treatment. Fasting urinary calcium/creatinine excretion was increased significantly (P less than 0.05) at the 3rd month, and plasma osteocalcin (OC) increased significantly from the 3rd month onwards (P less than 0.05) up to the 12th month (P less than 0.025). In the cross-sectional study, vertebral BMD was not significantly different from age-matched normal values in patients receiving either substitutive or suppressive doses of LT4. These results suggest that in the case of primary hypothyroidism even appropriate thyroid replacement therapy could lead during the first year of treatment to a significant reduction in vertebral and femoral BMD. However, the fact that an increased fracture rate has not been documented in long-term treated patients, and the results of our cross-sectional study, suggest that this bone mass reduction could be transient and reversible due to new bone formation at the end of the resorptive sequence.