THYROXINE SUPPRESSIVE THERAPY DECREASES BONE-MINERAL DENSITY IN POSTMENOPAUSAL WOMEN

OBJECTIVE Hyperthyroidism is associated with increased bone turnover and decreased bone mass. This study aimed to evaluate the bone mineral density (BMD) of post-menopausal women on long-term thyroxine suppressive therapy. DESIGN An age and sex-matched cross-sectional study. PATIENTS Thirty-four post-menopausal women with carcinoma of thyroid, post total thyroidectomy and I-131 ablation, on L-T4 for 12.2 +/- 6.6 years (mean +/- SD). Controls were 34 age-matched healthy Southern Chinese women. MEASUREMENTS Total body and regional BMDs were determined by dual-energy X-ray absorptiometry. Bone turnover was assessed by biochemical markers. RESULTS In the thyroxine treated group, total body mineral content was significantly lower than the controls (1652 +/- 356 vs 1994 +/- 270 g mean +/- SD, P < 0.005). They also had lower BMDs in the lumbar spine, femoral neck, trochanter and Ward's triangle (0.75 +/- 0.15 vs 0.92 +/- 0.16 g/cm2, P < 0.005; 0.62 +/- 0.12 vs 0.70 +/- 0.12 g/cm2, p < 0.01; 0.55 +/- 0.14 vs 0.63 +/- 0.15 g/cm2, P < 0.001; 0.55 +/- 0.14 vs 0.63 +/- 0.14 g/cm2, p < 0.005 respectively.) The thyroxine treated group also had higher serum alkaline phosphatase and osteocalcin levels as well as urinary hydroxyproline excretion, suggesting that they had high turnover bone loss. However, the Z-scores of the various regional BMDs were correlated only with the serum osteocalcin level and showed no correlation with the serum thyroxine level or with the dosage or duration of thyroxine treatment. CONCLUSION Long-term thyroxine suppressive therapy was associated with bone loss and preventive therapy may be indicated in these post-menopausal women at risk of osteoporosis.