Association between asthma and thyroid function as well as thyroid hormone sensitivity indicators: an NHANES study.

Objective: Thyroid hormones significantly influence multiple physiological systems, particularly the respiratory system. Despite limited research on asthma-thyroid associations, emerging studies have begun exploring this link. This cross-sectional study investigates relationships between asthma, thyroid function, and thyroid hormone sensitivity in U.S. adults using NHANES data.

Methods: A total of 8160 participants were included in this study. Weighted analyses of data from the 2007-2012 National Health and Nutrition Examination Survey (NHANES) were performed to examine the associations between asthma, thyroid function, and thyroid hormone sensitivity indices. Subgroup analyses and ROC curve investigations were also conducted. Additionally, a retrospective cohort of 30 asthma patients and 30 non-asthmatic controls seen at Zhongshan Hospital, Xiamen University between July 2023 and July 2025 was extracted; FT3 levels were compared and asthma control was evaluated across FT3 tertiles. Thyroid function parameters assessed were as follows: free triiodothyronine (FT3), free thyroxine (FT4), total triiodothyronine (TT3), total thyroxine (TT4), thyroid-stimulating hormone (TSH), thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), and thyroglobulin (Tg). Thyroid hormone sensitivity indices-namely, the FT3/FT4 ratio, thyrotroph T4 resistance index (TT4RI), thyroid-stimulating hormone index (TSHI), and thyroid feedback quantile index (TFQI)-were calculated from serum FT3, FT4, and TSH values.

Results: Asthmatics exhibited higher FT3 and FT3/FT4 levels but lower TPOAb than non-asthmatics. However, adjusted models (2 and 3) revealed an inverse association between asthma risk and FT3/FT4 (β: - 0.05, 95% CI - 0.09 to - 0.01). Quartile stratification maintained this inverse trend, with significant dose-response relationships in unadjusted Model 1 (P < 0.05). Subgroup analyses showed Mexican American asthmatics had lower FT3 and FT3/FT4 levels. ROC curves indicated superior predictive accuracy for TPOAb (AUC = 0.60) compared to FT3/FT4. Our institutional validation revealed that asthma patients had significantly lower FT3 levels than controls, and higher FT3 was associated with a lower proportion of acute exacerbations; however, the trend did not reach statistical significance.

Conclusion: FT3 and FT3/FT4 levels may inversely correlate with asthma risk, though causality remains unclear due to study design limitations. Further research is warranted.