Feline Hyperthyroidism

Feline hyperthyroidism causes cats’ thyroid glands to manufacture excessive amounts of thyroid hormone, also called T4.

The primary negative effects are elevated blood pressure.

Even previously-docile cats may be unable to rest because of hyperactivity resulting from feline hyperthyroidism.
Even previously-docile cats may be unable to rest because of hyperactivity resulting from feline hyperthyroidism.

That, in turn, commonly results in kidney damage and retinal hemorrhages. Often the retinas will detach during this process, leading to sudden blindness.


 As of this writing, no one knows why the syndrome has appeared in cats. It has been recognized for several decades and cases are on the increase. Many
theories have been proposed, but none has been proven.


 Cat owners may first notice their pet’s appetite increasing while its weight drops. The syndrome has been called “the case of the lean and mean cat.” Think of thyroid hormone as the “accelerator pedal” of the body. As levels of T4 increase, metabolic rate rises concomitantly. Along with blood pressure, heart rate goes up, cardiac output (blood flow volume) goes up, energy consumption goes up and excitability and anxiety may occur, making the previously-docile cat “mean.” Owners are perplexed about how their kitty can eat so much and still lose weight.

On the other hand, the first sign detected may be a lump in one or both thyroid glands. These are located in the throat, alongside the trachea (windpipe), just as in people. While lumps may be present but not producing excess hormone, that is the exception to the rule. Thyroid glands are routinely palpated for lumps in all cats, but especially those over six years of age. It is unusual to find hyperthyroidism below that age, and the most common age of onset is nearly twice that.


 Three main forms of treatment exist:

Oral or transdermal methimazole (formerly available as the trade name Tapazole and now veterinary-specific Felimazole) is the most widely-used therapeutic mode. It is safe, effective, affordable and tolerated by most cats. The only limitation is owners’ ability to pill the cat or rub transdermal medication onto an ear once or twice daily.

Surgical removal of lumps is the second-most-common form of treatment, and is within the grasp of most general practitioners with an interest in this procedure and willingness to learn. Surgery is curative, but not preventive. In other words, new lumps may form and require additional surgery.  Also, there is the risk of inadvertently removing the nearby parathyroid glands, which can be lifethreatening.

Radioactive iodine isotope131is another form of treatment, but requires the patient to be anesthetized for repeated treatments and a stay in a treatment facility until he is no longer excreting ionizing radiation. This is the most expensive form of treatment, but has the best long-term outcome with the least amount of aftercare.

Dietary therapy will likely be the treatment of choice for hyperthyroid cats now that Hill’s Pet Nutrition has released their Prescription Diet y/d.


 With all forms of treatment regular testing is needed to ensure thyroid levels stay within acceptable levels.

All patients need to have blood pressure monitored, and those whose pressure is elevated will need at least daily medication.

Critically important is following kidney function. Kidney disease is a common problem in the same age group of cats who suffer from hyperthyroidism. With elevated thyroid hormone increasing cardiac output and metabolism, the kidneys “see” and filter more blood than a cat with a normal metabolic rate. Once the level of T4 is reduced, the filtration rate of the kidneys is also reduced. Such cats may then suffer from insufficient kidney function, requiring that abnormality to be addressed.

See you Monday, Dr. Randolph.


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