Long-term L-thyroxine therapy is associated with decreased hip bone density in premenopausal women.

The effect of long-term L-thyroxine (L-T4) therapy on axial skeleton bone density was studied in 31 premenopausal women; the bone densities of these women were compared with the bone densities of 31 age- and weight-matched women without thyroid or bone abnormalities. The women receiving L-T4 therapy had been receiving the medication for a minimum of five years. There was no difference in calcium intake or excretion between the L-T4-treated women and the controls. Women receiving L-T4 had increased serum thyroxine concentrations (134 +/- 5 vs 95 +/- 3 nmol/L [10.4 +/- 0.4 vs 7.4 +/- 0.2 micrograms/dL]), an increased free thyroxine index (9.4 +/- 0.4 vs 6.8 +/- 0.2), and decreased serum thyroid-stimulating hormone concentrations (0.9 +/- 0.2 mU/L vs 2.1 +/- 0.3 mU/L [0.9 +/- 0.2 vs 2.1 +/- 0.3 microU/mL]). Serum triiodothyronine concentrations were normal and were similar in both groups. Women treated with L-T4 had a 12.8% lower bone density at the femoral neck and a 10.1% lower bone density at the femoral trochanter compared with matched controls. In contrast, lumbar spine bone density was similar in the two groups. The data suggest that long-term L-T4 therapy, which is often given at supraphysiologic dosages, may predispose patients to decreased bone density in the hip and may increase the risk of age-related bone loss. It is advisable, therefore, to employ a dosage of L-T4 that is carefully monitored to avoid the long-term use of dosages that are excessive for the thyroid condition being treated.