The thyroid glands are located in the neck area and play a vital role in regulating the body's metabolism. Hyperthyroidism is a common disease in older cats in which the thyroid glands produce too much metabolic hormone and, as a result, raise the body’s metabolic rate to unhealthy levels.
What are the clinical signs of hyperthyroidism?
The typical cat with hyperthyroidism is an older cat (i.e. >7 years). On history, most owners will report weight loss despite an increased/voracious appetite. In fact, some of these cats have such a ravenous appetite that they will literally eat anything in sight! Affected cats often drink a lot of water and frequently urinate. There may be a history of chronic vomiting or diarrhea, and the hair coat may be dull, dry, and unkempt. Because thyroid hormone controls metabolism, many organs are affected by hyperthyroidism including the heart and liver. The heart is forced to work harder to meet the increased demand for blood flow. This increased work load can cause the heart wall to thicken. Clinically, the extra stress on the heart may show up as an increased heart rate (tachycardia) and a heart murmur. Hypertension may develop as a consequence of the increased pumping pressure of the heart. In some cats, blood pressure can become so high that retinal hemorrhage or detachment will occur and result in blindness. Because the liver is the organ of metabolism, it is also affected by the increased thyroid hormone. The increased metabolic rate forces the liver to work harder. On blood work this generally shows up as increased levels of liver enzymes. Some of these kitties may vomit more often.
How is it diagnosed?
Although clinical symptoms may be suggestive of hyperthyroidism, hyperthyroidism is diagnosed by demonstrating high levels of thyroid hormone, generally thyroxine or T4, in a blood sample. When first diagnosing hyperthyroidism, addition tests, such as full geriatric blood work, urine analysis, and blood pressure readings, are also run to get a complete picture of organ function (including liver and kidneys) to provide the most accurate picture of your cat’s whole health before starting therapy.
What are the options for treatment?
Because less than 2% of these cats have cancerous growths of the thyroid gland, treatment is usually very successful. There are two main choices for treatment.
Oral medication: Methimazole is a medicine that can be given to “soak up” the excess thyroid hormone without actually destroying any thyroid tissue. Because no tissue is destroyed, this medication must be given every day for the rest of the cat's life. As dose requirements can change over time, periodic blood tests (generally ~6 months) must be done to keep the dosage regulated. To help decrease the stress associated with daily medication, Methimazole can be ordered in a variety of forms including tablet, flavored suspension, or ear gel.
Radioactive iodine: Radioactive iodine therapy (I131) is an effective treatment of hyperthyroidism because >99% of all Iodine is found in the thyroid. Given that iodine is preferentially found in the thyroid, when the radioactive form is absorbed, it radioactively destroys the abnormal thyroid tissue without endangering other organs. Radioactive iodine therapy is available in Calgary and is done as a referral appointment. For more information on I131 treatment, click here to be redirected to the referral center's website.
What is the long term prognosis for a hyperthyroid cat?
With proper treatment, the prognosis for hyperthyroidism is generally good. Factors that might affect the success of treatment include concurrent diseases, especially kidney disease, and your cat’s overall health.