Objectives. This study aims to evaluate the need to modify the total and weight-adjusted doses of levothyroxine after bariatric surgery, identify predictors, and assess the influence of the weight loss on the levothyroxine requirements. Methods. A retrospective study in patients with treated hypothyroidism that underwent bariatric surgery. The modification of the levothyroxine dose and its association with the weight loss and other potential predictors were evaluated at 6, 12, and 24 months post-surgery. Results. Among the 63 patients included, 82.54% needed an adjustment of levothyroxine dose during the follow-up. The total weekly dose of levothyroxine (µg) decreased post-surgery at 6 months (β= −49.1; 95%CI-93.7 to −4.5; p=0.031) and 12 months (β=−54.9; 95%CI-102 to −7.8; p=0.022), but did not significantly change at 24 months (p=0.114). The weekly weight-adjusted dose (µg/k) increased at 6 months (β=1.37; 95%CI 0.91 to 1.83; p<0.001), 12 months (β=2.05; 95%CI 1.43 to 2.67; p<0.001), and 24 months (β=2.52; 95%CI 1.74 to 3.30; p<0.001). The weight loss showed association with the weight-adjusted dose (OR=1.07; 95%CI 1.02 to 1.12; p=0.004), but not the total dose (p=0.320). Conclusions. This study shows a significant decrease in the total dose of levothyroxine requirements change after bariatric surgery during the first year of the follow-up and an increase in the weight-adjusted dose over the first two years. No predictors of modification of the total dose of levothyroxine were identified.

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