Summary A five-year-old female Guinea pig was presented for a history of 2 months losing weight despite an increased appetite. Physical examination revealed a loss of weight of 37% in 4 months, and a mass of 1 cm diameter in the cervical area. A thyroid hormones dosage revealed an increase of free T 4 and free T 3 . These results associated with the clinical signs lead us to conclude that this animal was suffering of hyperthyroidism. The animal was treated with methimazole (1 mg/kg bid). The owners reported a good tolerance to the treatment and a gain of weight of 25% in 1 month. Hyperthyroidism is not a pathology well described in Guinea pigs. Some diagnostics methods, such as dosage of thyroid hormones, scintigraphy, or advanced imaging are available in this species. Medical treatment, surgery, or injection of radioactive iodine could be considered.