LAUSR

Background Thyroid cysts are rare in cats and poorly documented. Objectives To report distinguishing clinical features and treatment responses of cats with thyroid cysts. Animals Forty client‐owned cats. Methods Retrospective review of medical records for cats with thyroid cysts confirmed by scintigraphy, ultrasound, magnetic resonance imaging, or necropsy at 4 referral centers between 2005 and 2016. Signalment, clinical findings, diagnostic testing, treatment, and outcome were recorded. Results Cats ranged in age from 8 to 20 years with no apparent breed or sex predilection. 37 of 40 (93%) cats were hyperthyroid (duration, 1–96 months). Clinical findings included palpable neck mass (40/40, 100%), weight loss (15/40, 38%), dysphagia (8/40, 20%), decreased appetite (5/40, 13%), and dyspnea (4/40, 10%). Cysts were classified as small (≤8 cm3) in 16 (40%) and large (>8 cm3) in 24 (60%) cats. Of 25 cats treated with radioiodine, hyperthyroidism resolved in 23 (92%), whereas thyroid cysts resolved in 12 (50%). Radioiodine treatment resolved small cysts in 8 of 13 (62%) cats and large cysts in 4 of 11 (36%) cats. Eight cats, including 2 euthyroid cats, underwent thyroid‐cystectomy; 3 with bilateral thyroid involvement were euthanized postoperatively for hypocalcemia. Excised cystic thyroid masses were identified as cystadenoma (4) and carcinoma (4). Conclusions and Clinical Importance Thyroid cysts are encountered in hyperthyroid and euthyroid cats with benign and malignant thyroid tumors. Radioiodine treatment alone inconsistently resolved thyroid cysts. Thyroid‐cystectomy could be considered in cats with unilateral thyroid disease or when symptomatic cysts persist despite successful radioiodine treatment of hyperthyroidism.