Thyroid Hormone Replacement Dosing after Bariatric Surgery in Patients with Primary Hypothyroidism And Severe Obesity: Tehran Obesity Treatment Study.

Background: Obesity and hypothyroidism are common medical conditions that are associated with each other. Bariatric surgery (BS) is a common approach used to achieve substantial weight loss in obese patients. However, there is limited evidence regarding the need for postsurgery levothyroxine (LT4) dose adjustment in patients with hypothyroidism undergoing BS. Methods: This was a three-year prospective cohort study assessing postsurgery LT4 requirements with attention to body composition changes. The current study included 1030 patients with hypothyroidism, who underwent sleeve gastrectomy (SG) (n = 707, 88.3% women) or one anastomosis gastric bypass (OAGB) (n = 323, 92% women). Patients were followed for 36 months after surgery. A bioelectrical impedance analyzer was used for body composition assessment. LT4 requirements were assessed by generalized estimating equation (GEE) methods adjusted for weight as a time-varying covariate. Results: During the follow-up, TSH (mIU/L) and T4 (ng/dL) measurements did not significantly change in the OAGB group over time. However, in the SG group, TSH measurement decreased over time (p_trend = <0.001). In the third year of the follow-up, 56.1% and 33.3% of patients in the SG and OAGB groups experienced LT4 (μg/day) dose reduction, while 24.4% and 9.1% of the participants experienced LT4 dose increments, respectively. GEE analysis showed a significant increase in the LT4/fat mass (FM) (μg/kg) ratio after 36 months of follow-up compared with the baseline in both the SG [1.8 (1.5-2.2) to 2.7 (2.0-3.5), p_trend = 0.039)] and OAGB [1.7 (1.4-2.2) to 3.2 (2.7-4.8), p_trend = <0.001)] groups. Moreover, patients who underwent OAGB experienced greater LT4/FM (μg/kg) dose adjustments compared to those undergoing SG (p_between = 0.060). In both groups, after the first year, the increase in LT4/FM (μg/kg) plateaued (p_interaction = 0.009). Conclusion: Most hypothyroid patients experienced either a reduction or no change in LT4 (μg/day) dosage after 36 months in both surgical groups. The LT4/FM (μg/kg) was significantly increased in patients undergoing either SG or OAGB with greater alterations in the latter. Further studies on larger populations and with longer duration of follow-up are needed to confirm our results.