Tumors derived from C-cells (parafollicular, ultimobranchial cells) of the thyroid gland are most common in adult to aged bulls and horses and in certain strains of laboratory rats. A high percentage of aged bulls has been reported to develop C-cell tumors (≥30%) or hyperplasia of C-cells and ultimobranchial derivatives (≥15–20%). These have not been seen in cows fed similar diets. The incidence in bulls increases with advancing age and is often associated with development of increased vertebral density. Multiple endocrine tumors, especially bilateral pheochromocytomas and occasionally pituitary adenomas, are detected coincidentally in bulls with C-cell tumors. A high frequency of thyroid C-cell tumors and pheochromocytomas has been reported in a family of Guernsey bulls, which suggests an autosomal dominant pattern of inheritance. A diffuse or nodular hyperplasia of secretory cells in the adrenal medulla often precedes the development of pheochromocytoma.