FT3 Levels and Systemic Inflammation: Evidence From a Population-Based NHANES Analysis.

Objective: Previous studies suggest a complex interaction between 3,3',5-triiodo-L-thyronine (T3) and inflammation, but this relationship remains unclear. This study investigates the association between free triiodothyronine (FT3) levels and inflammatory markers in the US population using National Health and Nutrition Examination Survey (NHANES) data. Methods: This study analyzed NHANES data from three cycles between 2007 and 2012, using Spearman correlation tests and multivariate linear regression. Subgroup analyses and interaction tests were conducted based on age, gender, race, body mass index (BMI), thyroid-stimulating hormone, and free thyroxine (T4) to determine the correlations between FT3 and seven systemic inflammatory markers: C-reactive protein (CRP), neutrophil-to-lymphocyte ratio (NLR), platelet-to-white blood cell ratio (PWR), platelet-to-lymphocyte ratio (PLR), monocyte-to-lymphocyte ratio (MLR), systemic immune-inflammation index (SII), and systemic inflammation response index (SIRI). Results: A total of 2306 participants were included in this study. Univariate correlation analysis showed that CRP, NLR, MLR, and PLR were significantly negatively correlated with FT3 levels (all p  < 0.05). After adjusting for confounders, FT3 levels were significantly negatively associated with CRP, NLR, and PLR (all p  < 0.05). Subgroup analysis showed that age significantly modified the associations between FT3 and systemic inflammatory markers, such as CRP, NLR, MLR, SII, and SIRI (p-interaction < 0.05). The inverse associations were more consistent among individuals aged ≥ 65 years. Conclusion: In the general population, FT3 levels exhibit inverse associations with systemic inflammatory markers.