Association between the FT3/FT4 ratio and thyroid nodules in euthyroid patients with type 2 diabetes: a cross-sectional study

The association between the free triiodothyronine-to-free thyroxine (FT3/FT4) ratio and thyroid nodules (TNs) remains unclear. This cross-sectional study examined this association in euthyroid patients with type 2 diabetes mellitus (T2DM). Between 2021 and 2023, we enrolled 934 patients with T2DM at Tianjin Medical University General Hospital. All participants underwent anthropometric assessment, physical examination, laboratory testing, and thyroid ultrasonography. Logistic regression models were used to evaluate the association between the FT3/FT4 ratio and TNs. Restricted cubic spline analysis was performed to explore potential nonlinearity. Overall, 730 participants (78.2%) had TNs. The prevalence was higher in women than in men (84.2% vs. 73.9%). Participants with TNs had lower 24-h urinary uric acid excretion, estimated glomerular filtration rate, glycated hemoglobin, and FT3/FT4 ratio than those without TNs. The between-group difference in the FT3/FT4 ratio was statistically significant (P = 0.027). In the unadjusted analysis, a higher FT3/FT4 ratio was associated with lower odds of TNs. However, the association was attenuated after adjustment for age and sex and remained nonsignificant after further adjustment for clinically relevant covariates. Restricted cubic spline analysis indicated a significant nonlinear association between the FT3/FT4 ratio and TNs (P for overall association = 0.036; P for nonlinearity = 0.022), with higher estimated odds of TNs observed at the lower end of the FT3/FT4 ratio distribution. However, the 95% confidence intervals included 1 across most of the distribution. In euthyroid patients with T2DM, the FT3/FT4 ratio was not independently associated with TNs under a conventional linear assumption. However, the spline analysis suggested a possible nonlinear relationship that warrants confirmation in prospective studies.