RESUMEN
Fistulas of the fourth branchial pouch have an external opening in the neck and the inner opening at the apex of the pyriform fossa. The tract passes from the left lobe of the thyroid, resulting in acute suppurative thyroiditis in most cases. Actinomycosis is an indolent, slowly progressive infection caused by anaerobic or microaerophilic gram-positive bacteria, primarily of the genus Actinomyces. These bacteria are filamentous with branching and may colonize in the oral cavity. Actinomycosis is a chronic disease characterized by abscess formation, tissue fibrosis, and draining sinuses. We experienced a case of 6-year-old boy who had presented with a left neck mass. Neck sono showed an approximately 3 cm-sized low echoic mass in the left thyroid gland. Thyroid scan was compatible with thyroid hypofunction. Gomori-methenamine silver stain after fine needle aspiration showed colonies of bacteria, are composed of long, thin, filamentous bacteria. Barium esophagogram showed a linear barium-filled track at the left pyriform sinus. Neck excisional biopsy was consistent with the remnant of a fourth branchial cleft fistula. We report a case of actinomycotic thyroiditis in a child with fourth branchial cleft fistula, with a brief review of related literature.