Association between FT3/FT4 ratio and obesity in euthyroid patients with type 2 diabetes: a cross-sectional study with mediation and threshold analyses

Background: Thyroid hormone sensitivity, reflected by the free triiodothyronine-to-free thyroxine ratio (FT3/FT4), plays a critical role in metabolic regulation. This study aimed to investigate the association between FT3/FT4 and obesity in euthyroid patients with type 2 diabetes. Methods: A total of 1,408 euthyroid patients with type 2 diabetes were included. Multivariable linear regression, subgroup, mediation, and nonlinear analyses were performed to examine the associations between FT3/FT4 and body mass index (BMI). Results: FT3/FT4 was positively and independently associated with BMI in fully adjusted models (β = 7.03, 95% CI: 2.53–11.53, P = 0.0023). Subgroup analyses showed consistent associations across age, sex, and drinking categories, with no significant interactions. Mediation analyses indicated that waist circumference and visceral fat mediated 43.6% and 50.5% of the total effect, respectively, whereas subcutaneous fat was not significant. Nonlinear curve fitting revealed a threshold at FT3/FT4 = 0.41, below which FT3/FT4 was positively associated with BMI, while above it, the association plateaued. Conclusions: FT3/FT4 is independently associated with obesity in euthyroid patients with type 2 diabetes. Its partial mediation through central adiposity and threshold-dependent pattern suggest that FT3/FT4 may serve as a practical biomarker for early risk identification and personalized obesity management. Clinical trial number: not applicable.