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.

How does hypothyroidism cause PU/PD? The main circulating thyroid hormone, thyroxine, has a diuretic effect, in part due to the fact that it causes increased blood flow to the kidneys. This increased flow allows more blood to be filtered, therefore more urine to be produced. The increased urine production causes the cat to need to fight dehydration by drinking more water.

Clinical signs: The classic hyperthyroid cat is older (often over 10 years old), PU/PD, and losing weight in the face of an increased appetite. Owners may notice that the cat is more active, aggressive, or vocal than usual. The cat may be vomiting intermittently.

Physical exam: The cat is usually thin and often has a noticeable mass in the thyroid gland when its neck is palpated. Hyperthyroidism can cause high blood pressure and, ideally, the cat’s blood pressure should be checked; this step is often skipped, since a cat that is stressed by the vet visit is not likely to give a good reading, anyway. Also, a thorough eye exam is warranted, as increased blood flow from elevated thyroid hormones can cause one or both retinas to detach.

Diagnosis: A combination of clinical signs, a palpable thyroid nodule, and elevated thyroid hormone in lab work confirm the diagnosis of hyperthyroidism. For a cat in early stages of the disease, the hormone level may be inconclusive and may need to be checked again in a few months. In more advanced disease, the blood levels are typically unequivocal.

Kidney values should be checked, as concurrent renal disease is common in hyperthyroid cats. There is a rather complex relationship between the thyroid levels and kidney function. On one hand, pre-existing renal disease is often masked by hyperthyroidism because increased blood flow through the kidneys makes their function appear better than it actually is. On the other hand, that increased blood flow can actually cause physical damage to kidney tissue, initiating or worsening kidney disease.

Treatment: There are several options for treating hyperthyroidism:

  • Methimazole. The first place to start is usually methimazole (Tapazole), a medication that blocks the production of thyroid hormone within the thyroid gland. It is given orally or transdermally (applied to the skin of the ear). Some cats cannot tolerate methimazole and may develop severe bone marrow problems, vomiting, anorexia, and self-inflicted facial wounds; owners of these cats must explore other treatment options.The benefits of methimazole are two-fold: it is relatively easy to use and, if necessary, treatment can be stopped. For instance, if kidney disease becomes apparent on lab work, the dose of medication can be tailored to reduce thyroid hormones slightly while keeping kidney values as close as possible to the normal range.
  • Thyroidectomy. Surgical removal of the thyroid gland provides complete reduction in thyroid hormone levels. The primary benefit is that it is permanent. The downside is that it cannot be reversed if renal disease is present. It is also invasive and carries the risks associated with anesthesia.
  • Radioactive Iodine. I have an entire post devoted to this subject. But the benefits and risks are similar to those of thyroidectomy.

A strategy adopted by many owners is to treat with methimazole until concurrent renal disease is ruled out – then to select a permanent treatment.

Treatment Monitoring: Initially, the cat is started on a very low dose, which is increased slowly. Until a therapeutic dose of the medication is achieved, hormone levels need to be checked regularly. After levels stabilize in the normal range, monitoring can be done two or three times a year, unless the cat begins to show clinical signs again. Methimazole will not reverse thyroid gland hyperplasia, which may continue to progress, thus requiring increased doses of medication.

Hyperthyroidism is a very common disease in American cats – and the top differential when a PU/PD cat that losing weight comes into the clinic. So, if your older cat is eating more but losing weight, drinking and urinating more, and becoming more vocal, it may be time to check for hyperthyroidism.

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