Methimazole (Tapazole) is a typical first-line treatment for feline hyperthyroidism

One of the strange realities of veterinary medicine is that cats become hyperthyroid, while dogs become hypothyroid. Even stranger: some treatments for hyperthyroid cats may leave them permanently hypothyroid, which does not appear to cause health problems the way it does in dogs.

What causes it? Hyperthyroidism is caused when the thyroid gland overproduces thyroid hormones, either because it has a benign or malignant growth, or because it has undergone “adenomatous hyperplasia.” The latter is by far the most common cause and is a fancy way of saying “it got bigger and is producing lots of hormones.” The thyroid is a bi-lobed structure that sits on either side of the trachea; and the increase in size can affect one or both sides. More