Thyroid Disease part 2: Feline Hyperthyroidism

Previously I wrote about the thyroid gland and its function in the body, as well as the occurrence of hypothyroidism in dogs. Cats tend to get the opposite problem, where too much thyroid hormone is secreted. Usually the excess thyroid hormone secretion is associated with tumors of the thyroid glands, known as thyroid adenomas. Over 95% of thyroid tumors in cats are benign and generally the disease responds well to medication.

Feline hyperthyroidism is a disease most commonly seen in older kitties, usually over the age of 8. It affects male and female cats equally, and there are some studies that show that purebred cats are actually less likely to be diagnosed with it than are mixed breed / domestic breed cats.

Symptoms to watch for include:

  • Weight loss (possibly severe) in the face of an increased appetite
  • Increased thirst and urination
  • Increased activity or agitation
  • Panting or increased respiratory rate at rest
  • Vomiting or diarrhea
  • Poor hair coat or decreased grooming
  • Seeking cool areas / avoidance of warmth

On physical examination, your veterinarian will also often notice a very high heart rate and/or a new heart murmur because the high levels of thyroid hormone stimulate the heart to beat faster than normal. We may also detect high blood pressure, rupture of vessels of the optic retina on eye exam and small masses in the area of the thyroid behind the voice box on the neck.

Hyperthyroidism will be diagnosed via a blood test for the thyroid hormone thyroxine, also called T4. Usually this value will be very elevated in affected cats, however sometimes follow-up tests are needed. Your veterinarian will also likely do a complete blood count (CBC), serum biochemistry panel and urinalysis to evaluate the distribution of red and white blood cells and the function of the liver, kidneys and GI tract.

There are several treatment options available for hyperthyroid cats, and the best one depends on the individual pet, owner, and whether any secondary conditions, such as kidney disease, are present. Initial treatment usually involves oral medication with a drug called methimazole, which inhibits the manufacture of thyroid hormone. After a period of 2-4 weeks, your veterinarian will recheck your cat’s bloodwork to make sure that thyroid levels are decreasing and that the pet is doing better at home. When a good response to treatment has been established, then other treatment options may be considered.

Surgery to remove the affected thyroid glands can be curative. However, this is not commonly done because of the risk of removing the closely associated parathyroid glands, which help control calcium levels and are very difficult to see. Surgery can be very successful when only one thyroid gland is affected, but nuclear scanning at a referral hospital may be required to confirm that only one gland is hyperactive, and the remaining gland could later become affected.

Radioactive iodine treatment, also known as I131, has a very good prognosis for curing hyperthyroidism with minimal side effects and is available in Mount Pleasant. However, pets must not have any concurrent disease and will be hospitalized for at least 1 week while the radioactive iodine is excreted and properly disposed of.

Many owners, once their cat is well-regulated, will often choose instead to maintain them on methimazole for the rest of their lives, with once or twice daily treatment. For cats who cannot be given pills easily that are not good candidates for surgery or I131, a transdermal ointment can be compounded that can be applied the inside of the ear.

In hyperthyroid cats, the biggest risk associated with treatment has to do with the kidneys. Many older cats will develop decreased kidney function as they age. If the function of the kidneys falls below 30% of normal, we begin to see specific changes in the urine and blood values, and owners will start to notice symptoms of increased thirst and urination. At this stage, we say that cats have chronic kidney disease, or CKD (also called chronic renal insufficiency or chronic renal failure). With hyperthyroid cats, their increased heart rate sends more blood to the kidneys, which may help them function better than they normally would. Once that increased blood flow is decreased to normal levels by treating the hyperthyroidism, if a cat has been on the verge of CKD the loss of the extra blood may unmask underlying kidney disease. Your veterinarian may need to monitor blood and urine values frequently during the first few months of treatment for hyperthyroidism to watch for this development. In a few cats where CKD is unmasked, it may be in the best interest of the cat to remain slightly hyperthyroid, and medication levels may be adjusted accordingly.

We don’t know what causes hyperthyroidism in cats. Some researchers have shown a possible correlation with a compound found in the pop tops on many wet cat foods, called bisphenol A diglycidyl ether (BADGE), an epoxy resin used to help seal the cans. Other research points to a possible association with flame retardants found in mattresses, carpet pads and electronics called polybrominated diphenyl ethers (PBDEs). No conclusive research has been conducted thus far however, and it may be that since cats are living longer thanks to better nutrition and improved preventative veterinary care, we are seeing a higher frequency of what would normally be a disease of old age. Some experts point out that many cats with hyperthyroidism only eat dry food, and many cats exposed to PDBEs do not develop hyperthyroidism, so there are likely multiple factors, including genetics, at play in the development of the disease. Hopefully further research will give us more answers in the future.

Overall, the prognosis for most cats diagnosed with hyperthyroidism is good with appropriate treatment and regular monitoring. Although treatment will often lower the thyroid level quickly it may take 1-2 months after starting therapy for cats to return to their normal habits, hair coats and routines