Thyroid Disease and Cats

by catfood

In the early 1990s, a client brought in Lester, a 12-year-old male cat. Lester had been diagnosed with liver disease, and her veterinarian was attempting to persuade her to have a liver biopsy done to further characterize the illness. Because he was hesitant to put Lester through surgery, the owner sought a second opinion. A recent CBC (complete blood count) and urinalysis on the cat revealed normal results, but elevations in several liver parameters.


Lester was a scrawny, bony, and scruffy little cat. When I inquired about his appetite, his owner exclaimed, “His appetite is the reason I brought him to the vet in the first place!” I suspected hyperthyroidism when she told me Lester was losing weight despite eating like a pig.

Making the Diagnosis

Hyperthyroidism is the most common glandular disorder in cats. It is most commonly caused by an excess of circulating thyroxine, also known as T4, a thyroid hormone. Hyperthyroidism can affect either male or female cats, but it almost always affects older animals. Less than 6% of cases occur in children under the age of ten, with an average onset age of 12 to 13 years.

Weight loss and increased appetite are the most common clinical signs of this condition. Weight loss occurs in 95 to 98 percent of hyperthyroid cats, with the remaining 67 to 81 percent having a voracious appetite. There have been reports of excessive thirst, increased urination, hyperactivity, untidiness, panting, diarrhea, and increased shedding. Vomiting occurs in roughly half of all affected cats. Clinical signs are caused by increased T4 levels, which affect various organ systems.

A battery of tests is required because diabetes, inflammatory bowel disease, intestinal cancer, and chronic kidney failure all share some of the clinical signs of hyperthyroidism. A complete blood count (CBC), chemistry panel, and urinalysis cannot diagnose hyperthyroidism, but they can rule out diabetes and kidney failure. In hyperthyroid cats, the CBC and urinalysis may be normal, but the chemistry panel frequently shows elevations in several liver enzymes. The vast majority of hyperthyroidism diagnoses are based on a simple blood test that reveals elevated T4 levels. Lester’s first veterinarian failed to perform this test, resulting in a misdiagnosis of liver disease due to elevated liver enzymes.

Unfortunately, only 2% to 10% of hyperthyroid cats have normal T4 levels. One possible explanation is that T4 levels in mild cases can fluctuate between normal and abnormal. Concurrent illness also suppresses elevated T4 levels, lowering them into the normal or high-normal range and fooling the veterinarian into thinking the cat’s thyroid status is normal. Concurrent illness is fairly common in these geriatric cats, and diagnosing hyperthyroidism can be difficult in these cats.


Treating the Cat

There are several hyperthyroidism treatment options, each with advantages and disadvantages.

  • Oral antithyroid medication is used. Methimazole (trade name TapazoleTM) has long been the treatment of choice for feline hyperthyroidism. It is extremely effective at correcting the condition, often in two to three weeks. Unfortunately, 15% to 20% of cats will experience side effects such as vomiting, lethargy, blood clotting problems, jaundice, itching around the head and face, and, in rare cases, blood cell abnormalities. The majority of side effects are minor and go away on their own, but some do necessitate medication discontinuation. A lifetime of daily medication is required, which is inconvenient for owners of pill-resistant cats. CBC and T4 levels must be checked on a regular basis for the rest of the cat’s life.
  • Thyroid gland removal is performed surgically. There is no need for anesthesia or surgery, and most patients can be cured with just one treatment. Fortunately, most hyperthyroid cats have benign, well-encapsulated tumors that are easily removed. Surgery is usually curative, but in these elderly patients whose disease has affected their hearts and other organs, anesthesia can be difficult. Although surgery may appear to be costly, it is frequently less costly than years of oral medication and routine bloodwork rechecks.
  • Radioactive iodine iodine therapy This is the most sophisticated and advanced treatment option. When radioactive iodine is injected (usually through the skin), it concentrates in the thyroid gland, irradiating and killing the hyperfunctioning tissue. A benign tumor called a thyroid adenoma, which involves one or, more commonly, both thyroid glands, is the most common cause of hyperthyroidism. Radioiodine treatment was previously only available in specialized, licensed facilities, but many private treatment facilities are now available across the country. Depending on local or state laws, cats may need to be kept at the treatment facility for 10 to 14 days until the level of radioactivity in their urine and feces drops to an acceptable level. Furthermore, radioiodine therapy is prohibitively expensive. Although the price has dropped from around $1,200 to between $500 and $800, many cat owners still find it prohibitively expensive.

Lester had a very high circulating T4 level. Because the cat was only 12 years old when he was diagnosed, and his owner did not want to give him a pill twice a day for the rest of his life, she opted for surgical thyroid gland removal. Lester’s surgery went off without a hitch, and all of his clinical symptoms went away within a few days.

Dr. Plotnick is vice president of Bergh Memorial Animal Hospital.

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