Background and Objective: Patients with hypothyroidism require thyroid hormone replacement. Many studies have shown that bone density reduction is caused by suppressive doses of levothyroxine, but there are contradictory results regarding the effect of levothyroxine replacement dose (non suppressive). The purpose of this study was to investigate the effect of replacement doses of levothyroxine on bone mineral density in premenopausal females with primary hypothyroidism. Materials and methods: In this analytic case - control study, 55 females aged 30 to 50 years with primary hypothyroidism who received levothyroxine for at least two years, with TSH of 0.1 to 4.5 over their follow ups underwent densitometry using DXA method in spine and hip and then were compared with 55 subjects in the control group. Results: The mean differences in bone mineral density and Z-score of total femur zone, neck of femur and lumbar spine zone were significant in control and case groups. 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