Objective To investigate the effects of substitutive and suppressive doses of levothyroxine on bone metabolism in patients with differentiated thyroid carcinoma after surgery and 131I ablation. Methods The patients, who had received levothyroxine(L-T4) for at least 3 years for treating their differentiated thyroid carcinoma after surgery and 131I therapy, were classified into substitutive group and suppressive group according to the levels of serum free triiodothyronine (FT3), free thyroxine (FT4), thyroid-stimulating hormone (TSH). We compared the levels of FT3, FT4, TSH, serum parathyroid hormone (PTH), serum calcium (Ca), serum phosphate (P), serum alkaline phosphates (ALP) and Bone mineral density (BMD) to those of healthy volunteers well matched for sex, age, menopausal status, and body mass index (BMI). Results No significant differences were found in the bone density and biochemical parameters of bone metabolism of the subjects treated with substitutive or suppressive doses of L-T4 compared with the control subgroup. No significant differences were observed among the subgroups according to accumulative doses of 131I. No bone fracture was found in all the patients. Conclusion The substitutive and suppressive doses of L-T4 are safe and necessary for patients with differentiated thyroid carcinoma after surgery and 131I therapy. Such treatment for 3 years is not associated with increased risk of osteoporosis. Much longer term of follow up is still needed in patients receiving substitutive and suppressive doses of L-T4.