BACKGROUND: Subclinical hypothyroidism (SCH) has recently been acknowledged as an unconventional risk factor for coronary artery disease (CAD) and characterized by poor prognosis, which may be due to atherosclerotic plaque characteristics. We conducted this study to observe coronary plaque characteristics in coronary artery disease patients with concomitant SCH. METHODS: Patients with coronary artery disease were enrolled in the study and divided into an SCH group (patients, n = 26; plaques, n = 35) and a non-SCH group (patients, n = 52; plaques, n = 66). They were divided 1: 2 according to propensity-matched analysis including age, diabetes mellitus, gender, CAD severity and culprit vessel. Optical coherence tomography (OCT) imaging was performed on all patients, and images were analyzed by two independent investigators. Lipid-rich plaques (LRP), the precursor of vulnerable plaques, were defined as having more than one quadrant occupied with lipid pool. Maximum lipid arcs were simultaneously recorded. Fibrotic plaques and calcific plaques were also identified. The presence of coronary dissection, plaque erosion, thrombus, macrophage, calcific nodule, thin-cap fibroatheroma and micro channel were all noted. RESULTS: The ratio of LRP in SCH group was significantly higher than that in non-SCH group (54% vs. 30.3%, P = 0.037). That was the case as well for the maximum lipid arcs value (181.5° ± 61.6° vs. 142.1° ± 35.9°, P = 0.046). While thin-cap fibroatheroma (TCFA) was detected, no difference was identified between the two groups in either TCFA ratio (20% vs. 16.7%, P = 0.579) or fibrous cap thickness (57.5 ± 14.0 vs. 63.5 ± 10.7 µm, P = 0.319). Other OCT characteristics such as dissection, plaque erosion, thrombus, macrophage shadow and calcific nodule were also similar. CONCLUSION: Higher ratio of LRP with greater lipid arc in SCH patients may be related to the plaque instability and poor prognosis in CAD patients with SCH.