Multiple endocrine neoplasia 2B and an undescended thyroid gland

A 14month-old infant with multiple endocrine neoplasia 2B and an undescended thyroid gland had elevated plasma levels of calcitonin. Cervical exploration revealed a 5-mm solid and cystic structure at the expected site of each superior parathyroid gland. These were excised. Microscopically, they featured a medullary microcarcinoma, an empty cyst, a normal parathyroid gland and minute focus of mucus-producing glandular epithelium. The carcinoma was diffusely positive for calcitonin, focally positive for thyroglobulin, focally positive for keratin and CAM5.2, and showed mucus production. The epithelium lining the cyst was keratin and CAM5.2 positive and focally positive for thyroglobulin and calcitonin. These findings provide evidence for a fourth-fifth branchial pouch contribution to the follicular component of the normal thyroid and also an explanation for the rare mixed medullary and follicular carcinoma and mucus-producing medullary carcinoma.