We report a case of adenolipoma of the thyroid gland, which is very unusual because thyroid adenoma rarely contains mature adipose tissue. A 55-year-old Japanese woman presented with a 1-month history of a neck mass. A blinded fine-needle aspiration biopsy showed benign follicular cells with no adipose cells, and we performed a left hemithyroidectomy. The specimen measured 7.0 x 4.4 cm and weighed 59.9 g. The cut surface of the tumor revealed a solid yellowish mass, and hematoxylin-eosin staining showed that the solid tumor was well separated from the adjacent thyroid tissue by a thin fibrous capsule. The tumor was found to be follicular adenoma composed of epithelial cells arranged in monotonous microfollicular patterns, intermingled with mature adipose tissue. No adipose tissue was found outside the tumor, and there were no signs of amyloid deposits in the tumor or adjacent thyroid tissue. The pathological diagnosis was adenolipoma of the thyroid gland. We discuss the pathogenesis of this entity and review the previously documented cases.
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