The effects of suppressive doses of L-thyroxine on the appendicular and axial bone mineral content were followed for 12-36 months after total or subtotal thyroidectomy in 15 premenopausal women. Compared to age-matched controls, these patients had a more marked bone loss of the spinal bone mineral content (2.6 +/- 1.9% vs 0.2 +/- 1.2% per year). The changes in radial cortical bone density were not significantly different from the control group. We conclude that when a suppressive therapy with L-thyroxine is necessary the rate of bone loss should be monitored at regular intervals.