Impaired central thyroid hormone sensitivity is associated with severe insulin resistance in euthyroid patients with idiopathic inflammatory myopathies: a cross-sectional study

To investigate the association between thyroid hormone sensitivity indices and severe insulin resistance (IR) risk in idiopathic inflammatory myopathies (IIM). This retrospective cohort included 299 IIM patients with normal thyroid function. We used the FT3/FT4 ratio, thyroid-stimulating hormone index (TSHI), thyrotroph T3 resistance index (TT3RI), thyrotroph thyroxine resistance index (TT4RI), and thyroid feedback quantile-based index (TFQI) to represent thyroid hormone sensitivity. IR was assessed using the atherogenic index of plasma (AIP), triglyceride–glucose (TyG) index, and metabolic score for insulin resistance (METS–IR), with severe IR defined as the highest quartile. Logistic regression, restricted cubic spline analyses, and subgroup analyses were performed. A total of 299 euthyroid IIM patients were included (mean age 52.1 ± 12.9 years; 66.8% female). Patients with severe IR had shorter disease duration and higher BMI. After full adjustment, each 1-SD increase in TT3RI was associated with an 8% increased odds of AIP-defined severe IR (OR = 1.08, 95% CI 1.01–1.14, P = 0.016). After quartile stratification, TT3RI was positively associated with AIP-defined IR (Q3 vs. Q1: OR = 3.83, 95% CI 1.60–9.15, P = 0.003; Q4 vs. Q1: OR = 3.35, 95% CI 1.38–8.18, P = 0.008); the Q3 of TT4RI and TFQIFT3 was also associated with AIP-defined IR. For METS–IR-defined IR, significant associations were observed for the Q3 of TT3RI, TT4RI, TSHI, and TFQI FT3 , and for the Q4 of TFQI FT4 . No quartile group reached significance for TyG-defined IR. Restricted cubic spline analysis revealed linear dose–response relationships between TT3RI and AIP-defined IR (overall P = 0.010) and METS–IR-defined IR (overall P = 0.049). Subgroup analyses by sex, age, and BMI showed broadly consistent associations, with a significant interaction for the Q3 of TT3RI with METS–IR-defined IR in participants aged ≥ 60 years ( P for interaction < 0.05). This study provides evidence for an association between reduced central thyroid hormone sensitivity and IR in patients with IIM. Indices such as TT3RI, TT4RI, TSHI, TFQI FT3 and TFQI FT4 may serve as promising biomarkers for assessing metabolic risk in IIM patients, providing a basis for early identification of high-risk individuals and the development of personalized metabolic management strategies.