Impaired sensitivity to thyroid hormone was associated with increased continuous metabolic syndrome score.

Background and aim: The relationship between thyroid hormone sensitivity and overall metabolic risk remains unclear.

Methods and results: Using National Health and Nutrition Examination Survey (NHANES) 2007-2012 data, we validated the continuous metabolic syndrome (MetS) severity score model and calculated MetS scores. Weighted linear regression assessed associations between thyroid sensitivity indices and MetS scores. Restricted cubic splines (RCS) examined nonlinearity in sensitivity analyses and age-stratified analyses evaluated association stability across groups. Thyroid hormone sensitivity was significantly associated with the severity score of persistent metabolic syndrome (MetS). Compared with the first quartile, the MetS score in the fourth quartile of each sensitivity indicator was markedly increased: free triiodothyronine/free thyroxine ratio (FT3/FT4: β = 0.15, P = 0.01), thyroid stimulating hormone T4 resistance index (TT4RI: β = 0.15, P < 0.01), thyroid stimulating hormone index (TSHI: β = 0.15, P < 0.01), thyroid feedback quantile index based on FT4 and FT3 (TFQI_FT4: β = 0.11, P = 0.04; TFQI_FT3: β = 0.27, P < 0.001). Subgroup analysis by age demonstrated a stable association among participants aged under 50 years. In the 50-69 years age group, only TFQI_FT3 exhibited a significant trend (P = 0.02), whereas no significant association was observed among subjects aged ≥70 years. Among all thyroid sensitivity indices examined, only TT4RI exhibited a statistically significant nonlinear association with the MetS score.

Conclusion: Thyroid hormone sensitivity indices, particularly the TFQI_FT3, provide a straightforward tool for assessing metabolic disorder severity in individuals under 70 years, facilitating early identification and intervention of metabolic dysregulation in this population.