Follicular Cell Hyperplasia (Goitre), Adenoma and Adenocarcinoma of the Thyroid Gland in Fourlined Terapon (Pelates quadrilineatus): Clinical and Histopathological Study: 2022-2023.

Thyroid disorders can manifest in several forms, including follicular cell hyperplasia (goitre), follicular cell adenoma and follicular cell carcinoma. Following the detection of large and gradually increasing tumour-like masses in the gular region of wild fourlined terapons (Pelates quadrilineatus) housed in various tanks in a public aquarium in June 2022, September 2022 and October 2023, several fish displaying clinical signs were transported to the Ornamental Fish Clinic at the Faculty of Veterinary Medicine, University of Tehran. Upon clinical inspection, the fish had prominent tumour-like masses in the gular area, located beneath the operculum. Upon histological examination of the first series, the masses revealed thyroid well-differentiated hyperplastic follicles that contained colloid and were lined by cuboidal epithelial cells. There were no signs of cellular atypia or mitotic figures. The study of the second series also revealed the existence of cellular atypia, but still no mitotic figures. In the examination of the third series, apart from the earlier lesions, there was evidence of neoplastic cell invasion into adjacent structures such as the adipose tissue and subepithelial layer. In addition, there was moderate cellular pleomorphism and a limited number of mitotic figures. Although the termination of water ozonation and the addition of potassium iodide to the feed were effective in treating thyroid hyperplasia, the treatment for thyroid neoplasia was not successful. The present study provides the evidence of clinical and histopathological findings of follicular cell hyperplasia, adenoma and adenocarcinoma of the thyroid gland in fourlined terapon.