Impact of preoperative thyroid status on BMI change after lifestyle intervention and bariatric surgery: Results from the OBESEPI cohort

BackgroundObesity is a chronic multifactorial disease characterized by substantial interindividual variability in weight loss after lifestyle intervention and bariatric surgery. Thyroid hormones play a key role in energy homeostasis, but their influence on postoperative weight outcomes remains insufficiently studied. ObjectiveTo evaluate the association between preoperative thyroid status and changes in body mass index (BMI) after lifestyle intervention and bariatric surgery over a five-year follow-up. MethodsWe conducted a retrospective cohort study including adults with class II or III obesity enrolled in the Obesite Severe et Epigenetique (OBESEPI) study. All participants underwent preoperative lifestyle intervention followed by bariatric surgery. Thyroid status was classified as euthyroid or hypothyroid based on clinical and biochemical criteria. BMI was assessed at baseline and at nine postoperative time points over five years. ResultsAmong 435 included patients, 71 (16.8%) had hypothyroidism. Baseline BMI was similar between groups, whereas diabetes was more frequent in hypothyroid patients (52.1% vs 37.7%; p = 0.022). Hypothyroid patients had significantly higher BMI at 6-24 months after surgery, but differences were no longer significant beyond three years. BMI trajectories and magnitude of weight regain were comparable between groups. Higher preoperative TSH levels were independently associated with BMI regain (OR 1.32, 95% CI 1.00-1.72; p = 0.047). Higher baseline BMI, younger age, and female sex were also associated with greater BMI regain. ConclusionsHypothyroidism was associated with lower early postoperative weight loss but did not influence long-term BMI trajectories. Higher preoperative TSH levels were independently associated with BMI regain. KEYPOINTSO_LIPreoperative hypothyroidism is associated with reduced early weight loss during the first two years after bariatric surgery. C_LIO_LILong-term BMI trajectories and weight regain patterns are similar between hypothyroid and euthyroid patients beyond three years of follow-up. C_LIO_LIHigher preoperative TSH levels independently predict BMI regain. C_LIO_LIBaseline BMI, younger age, and female sex remain key determinants of the magnitude of BMI regain after bariatric surgery. C_LI