Aim: Rheumatic manifestations are common in patients with Hashimoto's thyroiditis (HT).Since previous reports on the prevalence of arthritis in this disease may have a rheumatology referral bias, we sought to establish the prevalence of undifferentiated inflammatory arthropathy (UIA) in unselected HT patients as seen in an endocrinology clinic.Methods: Cross-sectional study of 92 consecutive HT patients and no definite rheumatic disease from the Endocrinology Division, Hospital Universitario de Caracas diagnosed by the presence of anti-thyroid peroxidase antibodies (n = 68) or typical ultrasonographic findings (n = 24). Undifferentiated inflammatory arthropathy was defined as combination of morning stiffness and joint pain with ≥2 characteristics of inflammatory joint pain. The study was revised and approved by the Ethics Committee of our hospital and all patients signed an informed consent form. Results:Twenty-three patients (25%; 95% CI 16-34) met the criteria for UIA. Joints most commonly affected were the knees, hands and ankles and the most common pattern was oligoarticular (82.6%). In the multivariate analysis, variables associated to the presence of UIA were the presence of myalgia (odds ratio [OR] = 19.41; 95% CI = 2.38-158.38) andRaynaud's phenomenon (OR = 4.32; 95% CI = 1.01-18.60). No association was found with demographics, duration of disease, comorbidities or thyroid function status.