In a longitudinal cohort of 1,034 euthyroid adults followed for three years post-bariatric surgery, preoperative free triiodothyronine (FT3) emerged as a robust predictor of both successful weight loss (≥25% total body weight) and type 2 diabetes remission (fasting glucose <5.6 mmol/L, HbA1c <6.0% off medications). Sex-specific FT3 thresholds were identified: 4.75 pmol/L for women and 5.05 pmol/L for men for weight loss; 4.57 and 5.00 pmol/L respectively for diabetes remission. Women with FT3 below threshold showed disproportionately worse outcomes, and breakpoint analysis revealed FT3 plateaus with age while correlating linearly with fat-free mass index.
This finding carries real clinical weight. FT3 — the metabolically active thyroid hormone — governs basal metabolic rate, thermogenesis, and lean mass preservation, all central to post-surgical recovery and sustained fat loss. That even euthyroid patients (normal TSH, normal T4) show meaningful outcome differences based on FT3 levels challenges the adequacy of standard thyroid screening before bariatric procedures. The sex-specific thresholds are a particularly actionable contribution: women metabolize and convert thyroid hormones differently, and this study quantifies that divergence surgically.
Limitations include its single-center Chinese population, limiting generalizability, and the observational design preventing causal inference. Whether optimizing low-normal FT3 preoperatively via intervention could improve outcomes remains untested. Still, this is a clinically meaningful, paradigm-nudging finding — especially as bariatric surgery competes with GLP-1 agonists for patient selection.