A Case of Asymptomatic Riedel Thyroiditis with Follicular Adenoma in a Patient with a Multinodular Goiter: An Unusual Association

Riedel's thyroiditis (RT) is a rare inflammatory disease that results in fibrosis of the thyroid gland and invasion to the surrounding structures of the neck. Follicular adenoma (FA) of the thyroid is the most common benign neoplasm of the gland.A 42-year-old woman was referred to the outpatient clinic due to a multinodular goiter and thyroiditis. The patient was euthyroid and thyroid function tests were within normal limits. Thyroid antibodies (thyroid peroxidase antibody and thyroglobulin antibody) were high. Thyroid ultrasonography showed multiple iso-hypoechoic nodules and thyroiditis. Fine-needle aspiration cytology was performed, and it was consistent with 'suspicious for a follicular neoplasm' according to the Bethesda system. Due to the clinical findings, which included weight loss and sweating, and the cytological results indicative of a follicular neoplasm, the patient underwent a total thyroidectomy. The histopathological diagnosis was RT associated with FA. The patient was started on thyroid hormone (thyroxine) replacement therapy after surgery and was evaluated for additional fibrosis related to RT.To our knowledge, this is the first case of RT associated with FA in an asymptomatic patient with a multinodular goiter and high thyroid antibodies reported in the literature.