Nuclear Localization of Immunoreactive β‐Catenin Is Specific to Familial Adenomatous Polyposis in Papillary Thyroid Carcinoma

Thyroid carcinoma is the first symptom in some patients with familial adenomatous polyposis (FAP). We evaluated the cellular localization of β‐catenin in thyroid carcinomas associated (n=4) or not associated (n=173) with FAP, since loss of functional protein of the adenomatous polyposis coli (APC) gene leads to nuclear accumulation of β‐catenin in adenomas and carcinomas of the FAP colon. Immunoreactive β‐catenin was demonstrated at the cell membrane of glandular cells of the non‐neoplastic thyroid and non‐FAP carcinomas. On the other hand, cytoplasmic and nuclear accumulation of β‐catenin is specific to FAP‐associated papillary carcinomas. The abnormality in the APC/β‐catenin pathway is thus also important in FAP‐associated thyroid carcinoma, and β‐catenin immunohistochemistry is a feasible screening method to identify occult FAP in young patients with thyroid tumors.