Epithelial thyroid tumors in cows.

From 1964 to 1973, 370 tumors were collected from cows of unknown age. Ten (2.7%) of these were primary thyroid tumors. Three were malignant. The benign tumors were solitary encapsulated adenomas in the parenchyma with more or less defined trabeculae, tubular, and microfollicular pattern. One of the malignant tumors was a cystic papillary adenocarcinoma, and two were small cell carcinomas consisting of small, sometimes binuclear, pleomorphic cells.