A difficult diagnosis: a case report of combined Riedel's disease and fibrosing Hashimoto's thyroiditis.

Riedel's disease (RD) is a rare form of chronic thyroiditis, predominantly characterised by fibrosis which may involve neighbouring tissues and organs. Hashimoto's disease (HD), on the other hand, is fairly common. Clinical differentiation between these diseases is often difficult, and the results of diagnostic imaging, laboratory tests and cytology studies are often similar. We report the case of a female patient with Riedel's thyroiditis displaying clinical, laboratory and radiological traits of both diseases. A 44 year-old Caucasian female was diagnosed with hypothyroidism. A fine-needle aspiration biopsy was performed; the findings were suggestive of an exacerbated chronic inflammatory process. However, a small lymphocyte-derived malignancy could not be ruled out with certainty, and so the patient was referred for elective thyroidectomy. The microscopic features of both specimens did not meet the criteria of Hashimoto's thyroiditis. The immunohistochemical studies revealed few scattered B lymphocytes (CD20 positive) and numerous scattered T lymphocytes (CD3 positive). Finally, Riedel's thyroiditis with an intense inflammatory infiltrate composed of lymphocytes was diagnosed. Reaching a diagnosis was particularly difficult in this patient, since Riedel's thyroiditis, the fibrosing form of Hashimoto's disease and malignant tumours of the thyroid can show similar traits upon physical and histopathological examination. As the clinical data was indicative of Hashimoto's thyroiditis and there were partial histological criteria of two forms of thyroiditis, namely Hashimoto's and Riedel's, the very rare diagnosis of a combined disease was made. Dense B and T lymphocytes and some plasma cell infiltrates, as well as the destruction of thyroid follicles by fibrosis extending into surrounding tissues, were supportive of the eventual diagnosis. Differentiating between the histopathological and clinical presentation of both diseases in one patient is difficult, primarily due to the partial overlapping of their histopathological traits. In order to avoid a diagnostic error, close cooperation between the endocrinologist and pathologist is mandatory. It is our opinion that in our patient the two diseases existed separately, and their coexistence was most likely coincidental.