PU/PD Series: Feline Hyperthyroidism

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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

Should my Cat Have a Radioactive Iodine Treatment?


Chantal, after treatment

A friend’s cat was recently diagnosed as hyperthyroid; and my friend, an avid web surfer, found out about radioactive iodine (“I131“) as a treatment option. Her cat is 15 years old and in good body condition (meaning that she isn’t thin, as many hyperthyroid cats get). She wants to know if it’s a good therapeutic choice for her cat.  

My answer: A firm “maybe.”   

Hyperthyroidism in cats usually results from either benign tumors or hyperplasia (expansion) of the thyroid gland, which then produces too much thyroid hormone. This increase in circulating hormone can then lead to increased heart rate and high blood pressure, GI problems (appetite changes, diarrhea, vomiting), hyperactivity, heart disease (cardiomypathy), and kidney disease. These cats often lose weight and begin urinating excessively, which are the things that owners notice first. Left untreated, this is a progressive disease whose secondary effects usually become life threatening.    More