Effect of TSH-suppressive doses of levothyroxine on bone mineral density in Thai women.

Thyroid hormone stimulates both osteoblast and osteoclast. However, the effect on osteoclast exceeds that of osteoblast resulting in a decrease in bone mass. TSH-suppressive doses of levothyroxine (L-T4) with otherwise normal thyroid function, so-called subclinical hyperthyroidism, has been reported to cause a reduction in bone mass. However, the sites of bone loss vary among studies. Moreover, the effect of menopausal status on thyroid-hormone-induced bone loss is inconclusive. Ethnic and geographical differences may modify the skeletal response to thyroid hormone. In the present study, we looked at the effect of TSH-suppressive doses of L-T4 on bone mineral density (BMD) in Thai pre- and post-menopausal women. Subjects consisted of 27 Thai females aged between 23-79 years. Eighteen were premenopausal and nine were postmenopausal. All were attending the Thyroid Clinic at Ramathibodi Hospital and had been on at least 150 micrograms/day of L-T4 for the treatment of nodular thyroid diseases for more than 2 years with at least one TSH value during the follow-up period in the suppressive range. None of the subjects had a previous history of Graves' disease. BMD was determined by dual-energy X-ray absorptiometry. Data of 54 age-matched healthy controls were used for comparison. BMD values were converted to Z-scores before analyses. Data were expressed as mean +/- SEM. Compared to controls, postmenopausal women on long-term L-T4 had decreased BMD at anteroposterior spine (-0.69 +/- 0.20 vs 0.05 +/- 0.17, P < 0.01), femoral neck (-0.61 +/- 0.35 vs 0.18 +/- 0.24, P < 0.05), femoral trochanter (-0.64 +/- 0.37 vs 0.13 +/- 0.22, P < 0.05) but not at Ward's triangle. In contrast to the findings in postmenopausal women. there was no significant difference of BMD compared to controls in premenopausal women at the lumbar spine, Ward's femoral neck or femoral trochanter. We conclude that Thai postmenopausal women on long-term TSH-suppressive doses of L-T4 have reduced BMD at various skeletal sites which may increase fracture risks. TSH-suppressive doses of thyroid hormone should only be prescribed when appropriate and no longer than necessary to minimize this adverse effect of excessive doses of thyroid hormone on bone.