Dyspnoea and large‐volume pleural effusion secondary to a cyst involving the cranial mediastinum in a cat

Abstract

A 7-year-old female spayed domestic shorthair cat presented for dyspnoea and haemoptysis. Point-of-care ultrasound revealed pleural effusion. Fluid analysis noted proteinaceous fluid rich in small lymphocytes. A mediastinal mass was present on radiographs. CT revealed a fluid attenuating structure within the left thyroid gland. Right-sided thyroid tissue was not identified. A fluid attenuating structure with associated amorphous soft tissue was present within the mediastinum. Aspirates were consistent with cystic fluid. A surgical cystectomy was performed. Histopathology of the left thyroid structure revealed a cystic thyroid follicular adenoma. The cystic structure involving the cranial mediastinum was suspected to represent an ectopic parathyroid cyst. Alternative differentials such as a branchial remnant could not be ruled out. Immunohistochemistry of the ectopic endocrine tissue associated with the cyst was negative for thyroglobulin and calcitonin. Parathyroid hormone immunohistochemistry was unavailable for confirmation. Post-operative hypothyroidism and hypocalcemia were medically managed. Surgical cystectomy resulted in the resolution of pleural effusion.