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.

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