A Very Treatable Ailment - Hyperthyroidism in Cats

A Very Treatable Ailment - Hyperthyroidism in Cats

Although its cause is a mystery, most cats diagnosed with hyperthyroidism have an excellent prognosis. 

By Arnold Plotnick, DVM. 
Published in Cat Fancy, September 2014

When Mie S. brought her 15-year-old cat, Cookie, to my cats-only veterinary hospital, she was well aware that he had lost a substantial amount of weight. However, she was shocked when I announced the current reading on the scale. “Wow! Eight and a quarter pounds?” she exclaimed. “He used to weigh 16 pounds!” Knowing that the most common disorders that result in weight loss in geriatric cats are diabetes, hyperthyroidism and chronic kidney disease (CKD), I was already formulating my questions for her as I carried him back from the scale to the exam table.

Was he drinking a lot of water and urinating excessively? Yes, Mie said, he was. I wasn’t surprised, as all three of the illnesses mentioned above can present with excessive thirst and urination. How about his appetite? “Ravenous,” she said. Cats with CKD tend to have a decreased appetite, so kidney dysfunction moved a notch lower on my list. Cats with diabetes and hyperthyroidism tend to have good appetites. Not just good, though. Exceptionally good, like Cookie’s. While diabetes can occur in almost any age of cat, it classically hits cats in middle age, around 8 or 9 years old. Hyperthyroidism, however, is a geriatric cat disease, tending to affect cats around 13 or older. With Cookie being 15 years old, my mental list had hyperthyroidism first, diabetes second and kidney disease third.

Typically, you cannot feel the thyroid gland in a normal cat’s neck during a physical examination. In cats with hyperthyroidism, however, the gland enlarges and begins to drift downward on the neck. I carefully felt the groove between Cookie’s trachea and the jugular vein, and as I got a little lower … presto! I could easily feel an enlarged thyroid gland. Looks like Cookie read the textbook.

What Is T4?

Hyperthyroidism is the most common glandular disorder in cats. As noted above, it is mainly a disease of elderly cats, with cats typically being around 13 or 14 years of age at the time of diagnosis. Hyperthyroidism occurs when a tumor develops in one or both lobes of the thyroid gland. This causes the thyroid gland to secrete an amount of thyroxine (commonly abbreviated as T4), the main thyroid hormone in cats. Fortunately, 98% of these tumors are benign.

Excessive levels of T4 cause an increase in the cat’s metabolism, and cats start burning calories like crazy. They try to compensate by eating more food, but they usually cannot keep pace, and these cats will lose weight despite having an excellent appetite. Other clinical signs that might or might not be seen in hyperthyroid cats are excessive thirst and urination, vomiting, diarrhea, panting, restlessness or hyperactivity, and excessive vocalization at night.

The diagnosis is usually achieved by measuring the blood level of thyroxine. Most cats with overt signs of hyperthyroidism will have a T4 level above the high end of the reference range. If a cat is suspected of having hyperthyroidism but the T4 level falls in the high end of (but still within) the reference range, a more sensitive blood test, called the “free T4” (FT4) test, can be run. In most cases, the FT4 level will reveal the cat to have hyperthyroidism.

Treatment Pros and Cons

Untreated, hyperthyroidism can lead to heart failure and dangerously high blood pressure. Fortunately, the disorder is easily treated. There are currently four options for treatment.

The ideal treatment is the administration of radioactive iodine, as is done for people with thyroid tumors. This is usually performed at a referral center. An injection of radioactive iodine is administered under the skin. The radioactive iodine concentrates in the thyroid gland, and this destroys the tumor. The disadvantages of this treatment are the cost (at the time of this writing, the typical cost of treatment is somewhere between $1,500 and $2,000) and the need for the cat to stay at the treatment facility for approximately 4 to 10 days until the levels of radioactivity in the urine and feces have decreased to an acceptable level.

If you are unable or unwilling to pursue radioactive iodine therapy, anti-thyroid medication would be the next most likely treatment. The drug methimazole (brand names Tapazole and Felimazole), when administered twice daily, will prohibit the thyrid tumor from being able to produce thyroid hormone. The advantage of oral medication is that it is less expensive than radioactive iodine. A disadvantage is the need to give the drug twice daily. As most cat parents know, cats can be notoriously difficult to medicate. For cats that fight taking a pill, the medication can be compounded into a liquid formulation. For those cats who refuse to allow anything to be put into their mouth, the medication can be formulated into a gel that is smeared onto the hairless, inside part of the ear. The gel is absorbed through the skin, and it will lower the blood level of thyroid hormone. Another disadvantage is the potential for side effects. In some cats, methimazole will affect the bone marrow, causing either a low white count and/or a low platelet count. In some cases, methimazole will cause intense facial itchiness, and cats will frantically scratch at their faces.  Owners who opt for medical treatment will need to return to the veterinarian regularly for thyroid level evaluation and periodic adjustments in the cat’s methimazole dosage when necessary.

Surgery to remove the thyroid gland is curative; however, this option has fallen out of favor and is rarely performed, mainly due to the risks of anesthesia, the cost of the surgery and the potential for postoperative complications.

The most recent development in the treatment of hyperthyroidism is a therapeutic diet (Hill’sPrescription Diet y/d) that, when fed exclusively, will control the condition. The diet works by severely limiting the amount of iodine in the diet. Iodine is a major component of thyroxine. By severely restricting the intake of iodine, the production of excessive T4 is thwarted, and the levels drop down to normal. Gina Manes, a veterinary technician in New York City, has been feeding this diet to her elderly cat, Melon, for more than a year. “Melon loves the prescription diet,” Manes says, “and her hyperthyroidism is very well controlled on it.” Unfortunately, some cats do not share Melon’s enthusiasm for y/d and will not eat it, or they grow tired of it after a few weeks or months.

A Concurrent Condition

Chronic kidney disease is another disorder common to older cats. Not surprisingly, about 10 percent of hyperthyroid cats have concurrent CKD at the time the hyperthyroidism is diagnosed. Treatment of hyperthyroidism in cats with kidney disease poses an additional challenge for veterinarians.

Cats with hyperthyroidism often have elevated heart rates and increased cardiac output and blood pressure. The blood flow to the kidneys is enhanced. This might be beneficial in maintaining kidney function or delaying the progression of CKD in cats already diagnosed with it. However, correcting the hyperthyroidism reduces the blood flow to the kidneys, which might worsen or destabilize the cat’s condition. Therefore, when treating cats with concurrent hyperthyroidism and CKD, it is important to treat initially with a judicious dose of thyroid medication, and gradually bring the T4 level to normal, while closely monitoring the kidney values.

An Enduring Mystery

Although there are many theories, exactly why some cats develop hyperthyroidism while others do not has been one of veterinary medicine’s enduring mysteries. Until this puzzle has been solved, there remains no reliable way to prevent the disorder. Fortunately, hyperthyroidism is very treatable, and most cats diagnosed with hyperthyroidism have an excellent prognosis.