Comparison of Dosing Schemes for Thyroid Hormone Replacement Therapy After Thyroidectomy.

Overview

abstract

BACKGROUND: Current thyroid hormone replacement therapy (THRT) is built on weight-based standard calculation of dose. A novel Poisson regression model, which accounts for seven clinical variables, was recently proposed to improve accuracy of THRT. We aimed to compare the accuracy of estimated THRT dose to reach euthyroid and the difference in predicted dose between the Poisson (scheme A) and the weight-based standard (scheme B) in patients following total thyroidectomy for benign disease. METHODS: We retrospectively reviewed medical record of patients who underwent total or completion thyroidectomy for benign disease at a single institution between 2011 and 2019. The THRT dose was calculated using both schemes. We compared the difference between calculated THRT and prediction rates for optimal THRT dosing needed to achieve a euthyroid state between dosing schemes. Patients were evaluated for achieving euthyroid state, defined as TSH 0.45-4.5 mIU/L. We also compared dosing error rates (> 25 mcg over- and underdosing) between schemes. Prediction rates were compared by BMI tertiles to account for the effect of BMI extremes in achieving euthyroid state. The difference in predicted dose between schemes was calculated in both the total sample size and patients that met euthyroid. A measure of agreement, Kappa, was used to estimate agreement between dosing schemes. RESULTS: A total of 406 patients underwent total thyroidectomy for benign disease, with 184 having sufficient follow up data confirming euthyroid state. Of the 184 patients, 85.9% (n = 158) were women, 81% (n = 149) were Hispanic, and 56.5% (n = 104) were obese with a median BMI of 30.8 kg/m2. Scheme A resulted in a higher, but not statistically significant, accuracy rate (A: 60.3%, n = 111 versus B: 53.8%, n = 99; P = 0.21). Overdosing errors were lower with Scheme A (A:17.9% versus B: 32.1%; P = 0.0025) and less extreme > 25 µg (A: 17.9% versus B: 26.1%; P = 0.08). A trend in improved accuracy in patients with a BMI > 35 kg/m2 was noted (A: 46.9% versus B: 34.4%; P = 0.20). Scheme A also resulted in less overdosing errors in obese patients compared to Scheme B (A: 19.2% versus 45.2%; P = 0.0006). The average difference in predicted dose between schemes was an entire dose difference, mean of 16.0 µg and 15.8 µg for the total and euthyroid samples respectively. Furthermore, for the majority of patients the predicted dose did not match between the two dosing schemes for total and euthyroid samples, 76% (n = 311) and 76% (n = 141) respectively. In patients that achieved euthyroid, agreement between dosing schemes was low to moderate (Kappa = 0.360). CONCLUSIONS: Lower rates of overdosing were found for scheme A, particularly with obese patients. No statistically significant differences in predicted THRT dose was observed between schemes. The difference in predicted dose between schemes was on average 15 ug, correlating with an entire dose. The consideration of clinical variables other than weight (scheme A) when determining optimal THRT dosing may be of importance to prevent overdoses, with particular clinical relevance in patients with higher BMIs.