Hyperthyroidism is the most common endocrinopathy in cats, and most cases are caused by multinodular hyperplasia or follicular cell adenoma, although thyroid carcinomas occur infrequently. Hyperthyroidism is rare in dogs, and most cases are caused by functional thyroid carcinomas. There are case reports of canine hyperthyroidism secondary to exogenous sources, and rarely thyrotoxicosis can be seen with therapeutic doses of levothyroxine prescribed for hypothyroidism. A case of juvenile hyperthyroidism has been reported in a cat and a histopathological diagnosis of diffuse thyroid hyperplasia was made. This is the first reported case of functional eutopic thyroid tissue in a young dog. Histopathological examination reported bilateral hyperplastic change in the thyroid glands, similar to the case of feline juvenile hyperthyroidism.
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