How to calculate levothyroxine dosage after total thyroidectomy

Abstract

Aim: To prevent hypothyroidism or hyperthyroidism, it is critical to administer thyroxine replacement after a total thyroidectomy. Herein, the best formula for determining the initiating replacement dose of levothyroxine (LT4) after a total thyroidectomy was determined. Material and Method: In this retrospective study, all of the patients were initiated on 100 µg LT4 and titrated to within the reference range for thyroid function test (TFT) and free T4. Correlations to body weight (BW), ideal BW (IBW), body surface area (BSA), and body mass index (BMI) were calculated. A total of 67 adult patients underwent a total thyroidectomy for benign thyroid disease. Comparisons of the 5 different formulas in the literature and classical methods were made in calculating the LT4 dose estimation. Results: Correlations were seen between the LT4 dose and patient BW (r=0.445 P<0.005), IBW (r=0.438 P<0.005), BSA (r=0.472, P<0.005), and BMI (r=0.275, P<0.005). Comparisons of the 5 different formulas and the classical methods in the literature were made in calculating the LT4 dose estimation. We found that all of the formulas were statistically close to each other when the LT4 dose formulas described in the literature were compared. A regression equation was calculated (predicted LT4 dose=21.3+(0.46xBW)+(0.77xIBW) pragmatically. Discussion: Although the correct initial dosing of LT4 is still difficult, the formula we developed may increase the number of euthyroid patients over a shorter period of time postoperatively. The LT4 requirements of just the conventional formula (1.6 µg/kg), based on BW, were not statistically different, making it practical to use the weight-based calculation of the LT4 dose as a reasonable option using BW.