Objective: Currently, postoperative thyroid hormone replacement dosing is weight-based with adjustments and made following Thyroid Stimulating Hormone (TSH) values, which may lead to delayed achievement of euthyroidism and failure in achieving an accurate estimation of the levothyroxine dose. We aimed to evaluate the relationship between Body Mass Index (BMI) and Lean Body Mass (LBM) with levothyroxine dose.
Materials and Methods: Eighty patients with thyroid disease (benign and malignant) randomly enrolled the study. BMI and LBM were calculated during the pre-operative visit. Thyroid hormone replacement was started five day after surgery for benign and after a month for malignant cases, at a dose of 1.6 mcg/kg/day based on actual body weight. When euthyroidism was achieved, the levothyroxine dose was measured, and efforts were made to analyze its relationship with weight, BMI, and LBM.
Results: In the benign group, levothyroxine dose was obtained at 1.51 mcg/kg per body weight. To determine the levothyroxine dose based on BMI and LBM, Y=0.013+0.005 BMI and Y=0.048+0.002 LBM formulas were used, respectively. In the malignant group, a levothyroxine dose of 1.56 mcg/kg was obtained based on the weight variable. To determine the levothyroxine dose based on BMI and LBM, Y=0.01+0.004 BMI and Y=0.042+0.002 LBM formulas were used, respectively. In both groups, the levothyroxine dose that was calculated was more accurate using BMI with the obtained formula.
Conclusion: Weight, BMI, and LBM can be used to determine the levothyroxine dose in patients receiving total thyroidectomy, but levothyroxine can be precisely measured using BMI.