Using Body Mass Index to Predict Optimal Thyroid Dosing after Thyroidectomy

Abstract

Background Current post-operative thyroid replacement dosing is weight-based with adjustments made following TSH values. This method can lead to significant delays in achieving euthyroidism and often fails to accurately dose over and underweight patients. Our aim was to develop an accurate dosing method that utilizes patient BMI data. Study Design A retrospective review of a prospectively collected thyroid database was performed. We selected adult patients undergoing thyroidectomy with benign pathology who achieved euthyroidism on thyroid hormone supplementation. BMI and euthyroid dose were plotted and regression was used to fit curves to the data. Statistical analysis was performed using STATA 10.1 (StataCorp, College Station, TX). Results 122 patients met inclusion criteria. At initial follow-up, only 39 patients were euthyroid (32%). 53% of patients with a BMI>30 were overdosed, while 46 % of patients with BMI<25 were under-dosed. The line of best fit demonstrated an overall quadratic relationship between BMI and euthyroid dose. A linear relationship best described the data up to a BMI of 50. Beyond that, the line approached 1.1 mcg/kg. A regression equation was derived for calculating initial levothyroxine dose (mcg/kg/day = −0.018*BMI +2.13 {F statistic =52.7, root mean squared error of 0.24}). Conclusion The current standard of weight based thyroid replacement fails to appropriately dose underweight and overweight patients. BMI can be used to more accurately dose thyroid hormone using a simple formula.

Variation in the biochemical response to l-thyroxine therapy and relationship with peripheral thyroid hormone conversion efficiency

“…L-T 4 dose requirements have been well studied and various regimes based on weight, BMI or more refined algorithms have been proposed to put patients on a presumed adequate dose from the very beginning (10,11,12,13,14,15,16,35,36,37,38,39). As useful as these algorithms may be for average predictions and initial guidance in the general population, they do not take into account individual variations in the response to L-T 4 , such as conversion efficiency.…”

“…This parameter has evolved into the main treatment target to be monitored and kept within an assumed euthyroid range (9). A number of studies have attempted to predict T 4 requirement, and various regimes for a starting dose have been proposed based on an average of 1.6 mg/kg body weight (BW) or by more refined weight-or BMI-related algorithms (10,11,12,13,14,15,16).…”

Clinical practice guidelines for the management of hypothyroidism

“…In some clinical situations, more frequent monitoring is mandatory: (pregnancy (198) (B), significant changes in body weight (199) …”

How to calculate levothyroxine dosage after total thyroidectomy

“…Commonly, the initial dose of LT4 replacement is weight-based, recommended at a range of 1.6-1.7 µg/kg/day, with some reports of up to 2.1 µg /kg [2][3][4][5][6]. Some researchers have suggested a relationship between the replacement LT4 requirement and things like body mass index (BMI) or body surface area (BSA) [7,8], sex and menopause [9][10][11][12], age [11][12][13], and the disease resulting in hypothyroidism [3]. There is not much literature regarding surgically-induced hypothyroidism and thyroid hormone replacement, and the reports there are, lack a lone predictive factor with a strong relationship to accurately find a dosage of LT4 after surgery.…”

“…Hence, few methods in the literature have described an optimal calculation for the starting dose of LT4 following thyroidectomy. These are based on BMI, BW, age and sex [7,[13][14][15][16] (Table 1). The purpose of this study was to compare the various methods of T4 replacement described in the literature. )…”