Low Preoperative Free Triiodothyronine, Particularly in Females, Is Associated With Reduced Weight Loss and Diabetes Remission After Bariatric Surgery in Euthyroid Adults.

AIMS: With the rising prominence of GLP-1 receptor agonists, identifying predictors of optimal bariatric surgery candidates has become increasingly important. We investigated cross-sectional relationships between thyroid hormones and obesity-related parameters, and evaluated the predictive value of baseline thyroid hormones for weight loss (WL) and diabetes remission (DR) after bariatric surgery. METHODS: This study included a cross-sectional cohort of 1567 euthyroid patients with overweight or obesity, of whom 1034 with complete 1 year postoperative data were included in the longitudinal analysis. Body composition was assessed using validated predictive equations and abdominal MRI. The primary outcomes were successful WL (total WL ≥ 25%) and DR (fasting glucose < 5.6 mmol/L and HbA1c < 6.0% without antidiabetic medications) during the 3 year follow-up. RESULTS: Breakpoint analysis indicated that free triiodothyronine (FT3) plateaued beyond an age threshold while showing a linear correlation with fat-free mass index. Higher baseline FT3 tertiles were significantly linked to increased likelihood of achieving successful WL and DR at 1 year follow-up, with stronger associations in females. Optimal FT3 thresholds were identified as 4.75 (females) and 5.05 (males) for WL, and 4.57 (females) and 5.00 (males) for DR. Preoperative FT3 levels below these thresholds were significantly associated with reduced WL and DR over 3 years. CONCLUSIONS: Serum FT3 exhibits an age-related breakpoint effect, and low preoperative FT3, particularly in females, is a critical predictor of suboptimal short- and long-term WL and DR success post-bariatric surgery. Sex-specific FT3 thresholds may aid in preoperative risk stratification and inform personalised treatment decisions in the era of expanding obesity therapeutics.