Levothyroxine Dosing after Total Thyroidectomy and Radioactive Iodine Treatment: Development and Validation of a Unified Predictive Formula

Background and Objectives: Levothyroxine (LT4) replacement is required following total thyroidectomy (TT) for differentiated thyroid cancer (DTC), with the need and degree of thyroid-stimulating hormone (TSH) suppression determined by recurrence risk.Excessive or insufficient suppression increases cardiovascular, skeletal, and recurrence risks.Nonetheless, optimal LT4 dosing to achieve target TSH remains uncertain.This study aimed to develop and validate a formula to predict individualized LT4 requirements after thyroidectomy.Materials and Methods: Using data from 444 DTC patients who underwent TT and radioactive iodine therapy at Seoul National University Hospital, multiple linear regression identified body weight, height, age, sex, and TSH as predictors of daily LT4 dose (g/day).A unified model ("Formula 1") was compared with the American Thyroid Association (ATA) weight-based recommendation (2.1-2.7 g/kg).Results: The mean LT4 dose was 12638 g/day (1.9 g/kg).Body surface area demonstrated the strongest correlation with LT4 dose (r=0.481).In multivariable analysis, body weight and height positively associated with LT4 requirements, whereas age, female sex, and TSH associated inversely.The proposed formula exhibited a higher correlation with actual LT4 dose compared to the ATA method (r=0.574 vs. 0.462), as well as a lower prediction error (17.715.0 vs. 23.619.9g/day). Conclusion:The new multivariable formula reduced prediction error compared with conventional weight-based dosing, particularly among patients requiring stronger TSH suppression.By incorporating demographic and biochemical factors, this approach enables more precise LT4 adjustment and demonstrates clinical utility for model-informed individualized therapy.