Objective:The objective of this study was to optimize dose of levothyroxine (LT4) based on lean body mass (LBM) in young athyrotic patients with differentiated carcinoma of thyroid (DCT) which has not been properly addressed in Bangladesh before.Materials and Methods:Sixty patients with DCT (age, range: 20-39 years) having total thyroidectomy followed by radioiodine ablative therapy (RIT) and 23 euthyroid volunteers were recruited. Clinical, biochemical parameters were obtained from all patients after 2 months of RIT and on LT4 replacement at a dose of 200 μg/day as first follow up visit and also from control subjects. Then 60 patients were divided into two groups consisting of 30 patients each. Patients of Group-I received LT4 replacement based on LBM measured by dual energy X-ray absorptiometry (DXA) and Group-II continued LT4 replacement in conventional dose. Patients of both groups were assessed again for same parameters at 6 to 12 months at the second visit.Results:Optimized dose of LT4 based on LBM by DXA (131 ±23 μg/day) significantly reduced thyroid hormones and kept thyroid stimulating hormone (TSH) in expected levels in patients of Group-I at the second visit compared to patients of Group-II who continued conventional LT4 dose (200 μg/day). Hyperthyroid symptom scale (HSS) was significantly reduced to 2 ± 1 in patients of Group-I but not in patients of Group-II, HSS, 8 ±1 (P > 0.001).Conclusion:Optimization of LT4 dose based on LBM can avoid chronic exposure of mild excess of thyroid hormone in young patients with low risk DCT.