OBJECTIVE Studies of the effect of thyroxine therapy on skeletal integrity have given conflicting results; the reductions in bone mass reported by some have prompted recommendations that the prescribed replacement doses of thyroxine should be reduced. We have examined bone mineral density in a group of patients with differentiated thyroid carcinoma receiving high doses of thyroxine to suppress thyroid stimulating hormone (TSH). METHODS The 44 patients (6 male, 38 female) had a median age of 49 years (range 27-75) with median duration of thyroxine therapy of 9.0 years (range 3 to 42) and mean dose of thyroxine 0.167 mg/day (range 0.125-0.3). TSH levels were chronically suppressed in 39 subjects. Bone mineral density (BMD) was measured by dual energy x-ray absorptiometry (DEXA) in all subjects at the femoral neck and lumbar spine and compared with previously established local reference ranges. RESULTS There was no reduction in bone mineral density in the thyroxine treated group compared with the local reference population at both lumbar spine and femoral neck, and no correlation with duration of therapy. CONCLUSIONS These negative findings, that thyroxine in suppressive doses does not significantly reduce bone mineral density in New Zealand patients suggest that thyroxine therapy alone is not a major risk factor for the development of osteoporosis.