What is Hyperthyroidism?
Hyperthyroidism is the most common
endocrine disorder in cats, and is one of the most common diseases of older cats.
Most cats diagnosed with hyperthyroidism are over 10 years old. Our cat Barney was treated for both
hyperthyroidism and diabetes, and it's not unusual for a cat to have both
Typical signs of hyperthyroidism include
In addition to the signs listed above, hyperthyroidism causes
Untreated hyperthyroidism can lead to
The levels of thyroid hormone in the body are measured by a blood test. Other blood tests and methods of imaging the thyroid gland may be performed.
There are three treatment options available, and the method used will will depend on the pet.
Medication prevents the thyroid gland from producing excess thyroid hormones. Methimazole (Tapazole) is a commonly used antithyroid drug. Normal thyroid hormone levels are usually achieved in 1-3 weeks. Side effects include anorexia and vomiting, but these often lessen with time. It may be possible to decrease the dose to lessen the side effects while still maintaining proper thyroid hormone levels. Additionally, some cats may become hyperthyroid again, even while taking medication. Hospitalization is usually not required.
Surgery involves partial or total removal of the thyroid gland. Normal thyroid hormone levels are usually achieved in 1-2 days. Side effects include surgical risks, and hypothyroidism (low thyroid hormone). The parathyroid gland (a different gland next to the thyroid gland) may also be removed or damaged, resulting in hypoparathyroidism. Because hyperthyroid cats may have increased stress on their heart, they are usually started on antithyroid medication prior to surgery so the thyroid hormone levels are normal and the effects on the heart are minimized. The cat is hospitalized for 1-3 days.
Radioactive iodine treatment
Radioactive iodine is used to kill the overactive thyroid cells. Normal thyroid hormone levels are usually achieved in 1-12 weeks. This is considered the best treatment if a facility is available. It is very low risk, but not all cats are candidates, particularly cats with other health problems that require them to be handled frequently. After radioactive iodine treatment, the cats can't be handled right away because they must eliminate the radioactive iodine first. This makes it difficult for a diabetic cat, who requires insulin injections, to be a candidate for radioactive iodine treatment. Also, few facilities are available for this treatment, and the cat must be hospitalized for 1-4 weeks.
Considerations for Diabetics
Since both hyperthyroidism and diabetes are common in older cats it is often necessary to
deal with both conditions. Usually, the cat has one disease, which is being treated,
then the second disease begins and must be dealt with.
Barney - diabetic, hyperthyroidism treated with Tapazole
Barney was diagnosed with hyperthyroidism 6 months after being diagnosed diabetic. The hyperthyroidism was detected during a routine pre-anesthesia blood test (he needed his teeth cleaned). His thyroid hormone levels were moderately elevated, and his heartbeat was faster than normal. The teeth cleaning was postponed because the hyperthyroidism was placing a strain on his heart and that meant he could not safely undergo anesthesia. Barney was started on Tapazole, 5 mg twice a day. Shortly after starting Tapazole therapy, he started vomiting several times a day. Since he is also diabetic, it is difficult to know if the vomiting was caused by the Tapezole, or if it was due to his diabetes not being well controlled, or due to some unknown reason. But vomiting is not something you want with a diabetic because you need to try to keep their food intake as consistent as possible. Barney's thyroid hormone levels were not extremely high, so the vet thought it would be ok to decrease his Tapezol dose to 1/2 a tablet twice a day. He showed an immediate improvement and the vomiting stopped. A follow-up blood test was done, and his thyroid hormone levels were in the low-normal range. Barney has continued on this low dose of Tapazole. Since the hyperthyroidism increased metabolism and had an effect on the diabetes, during the time when his thyroid hormones were returning to normal, we monitored his blood glucose levels to be sure he was not getting too much insulin.
This site is for information purposes only. Please consult your veterinarian.