This study aimed to investigate the associations between thyroid hormones, metabolic syndrome (MetS), and coronary artery stenosis (CAS) in euthyroid patients, given the insufficient evidence on concurrent evaluation of these endpoints. In this cross-sectional study, demographic data, biochemical parameters, and coronary angiography were collected. The Gensini score was quantified to determine CAS severity. Participants were assigned to non-MetS and MetS groups, and further categorized into non-CAS, CAS I (stenosis < 50%), and CAS II (stenosis ≥ 50%) subgroups. The association between thyroid hormones, MetS, and CAS was analyzed. This study was approved by the Ethics Committee of the Army Medical Center of PLA. A total of 947 euthyroid adults were enrolled in the present study. Notably, free triiodothyronine (FT3) and the FT3/free thyroxine (FT4) ratio were significantly higher in the MetS group compared with the non-MetS group (all P < .05). In contrast, FT3, total triiodothyronine, and the FT3/FT4 ratio were markedly lower in the CAS II group than in the non-CAS group (all P < .05). Spearman correlation analysis in CAS patients showed that FT3, total triiodothyronine, and the FT3/FT4 ratio were negatively correlated with the Gensini score, and positively correlated with diastolic blood pressure, body mass index, and triglycerides (P < .05). After adjustment for potential confounders, multivariate linear regression analysis showed that FT3 remained significantly and linearly negatively correlated with the Gensini score (P < .001). In euthyroid individuals, thyroid hormones, particularly FT3, have inconsistent correlations with MetS and CAS, reflecting the complexity of thyroid hormone physiology.