Background Obesity 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.

Objective To 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.

Methods We conducted a retrospective cohort study including adults with class II or III obesity enrolled in the Obésité Sévère et Épigénétique (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.

Results Among 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.

Conclusions Hypothyroidism 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.

KEYPOINTS

Preoperative hypothyroidism is associated with reduced early weight loss during the first two years after bariatric surgery.

Long-term BMI trajectories and weight regain patterns are similar between hypothyroid and euthyroid patients beyond three years of follow-up.

Higher preoperative TSH levels independently predict BMI regain.

Baseline BMI, younger age, and female sex remain key determinants of the magnitude of BMI regain after bariatric surgery.

The authors have declared no competing interest.

The OBESEPI study is registered at ClinicalTrials.gov (NCT02663388).

This study did not receive any funding

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The study protocol was approved by the Ethics Committee of Nancy UniversityHospital (CPP est III, number : 2015-A01175-44).

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

All data produced in the present study are available upon reasonable request to the authors