Hereditary hemorrhagic telangiectasia (HHT) is an autosomal-dominant disorder with incomplete penetrance characterized by vascular anomalies that can develop in many organs. The diagnosis is based on 4 clinical criteria: family history, epistaxis, mucocutaneous telangiectasias, and arteriovenous malformations. Hereditary hemorrhagic telangiectasia is associated with many autoimmune diseases. An association with thyroid disease has not been reported. In a patient with HHT, we first identified chronic autoimmune (Hashimoto) thyroiditis, which is an organ-specific autoimmune disease and accompanies many other autoimmune diseases. The diagnosis of HTT was made by the findings of a positive family history, multiple telangiectatic lesions, and recurrent epistaxis. Nevertheless, we identified Hashimoto thyroiditis in this patient by the abnormalities of thyroid hormones, serum autoantibodies, radiographic and physical findings. Clinical improvement was seen by using blood transfusions, oral ferrum, and levothyroxine sodium therapy. In conclusion, in the existence of recurrent epistaxis and telangiectasias or a family history of these symptoms, HHT should be taken into consideration, and also Hashimoto thyroiditis as well as other autoimmune diseases should also be screened in patients with HHT.