Black thyroid: a pitfall for aspiration cytology.

Black thyroid discoloration following long-standing use of minocycline has been reported. Morphologic findings of aspiration cytology of these lesions was first reported from The Ohio State University. This abstract describes a second case of black thyroid that was preceded by aspiration cytology. In both cases, thyroidectomies were performed based on "indeterminate" fine-needle aspiration cytology (FNAC). Degenerative changes in follicular epithelial cells in black thyroid causes nuclear hyperchromasia and chromatin clumping, which may be mistaken for neoplasia. Pigment present in follicular epithelial cells and macrophages may be obscured by pigments with similar microscopic appearances, such as hemosiderin. A clinical history of the chronic use of a tetracycline derivative should alert the pathologist to the possibility of black thyroid. Diagnosis may be made by applying special stains on the cell block. The pigment stains with Melanin stain (Fontana) and bleaches with potassium permanganate. In the light of increasing use of FNAC of the thyroid gland and the large number of patients who received tetracycline therapy in the last two decades, it is likely that some practicing cytopathologists may experience this pitfall.