Hyperthyroidism, excessive production of hormone from the thyroid gland, is a common disease in older cats. Symptoms of hyperthyroidism often come on very gradually and can include weight loss, vomiting, or diarrhea. Thyroid hormone is responsible for setting the body’s metabolic rate, and when there is excessive hormone production we see the effects on heart rate, energy level, and appetite. Often a client will bring in a senior cat and remark how active they are at home, how well they are eating, despite losing weight. In reality, these cats have excessive hormone that is causing an increase in appetite and energy level, while taxing the heart and eating away at muscle mass.
Ninety five percent of thyroid tumors in cats are benign. Once diagnosed, there are several treatment options available including twice daily medication and radioactive iodine.
The medication (methimazole) reduces the overproduction of the thyroid hormone but must be given continuously for the remainder of the cat’s life. Bloodwork is initially monitored monthly until they are determined to be on an appropriate dose of medications, then every 6 months. The medication can in very rare cases affect the bone marrow or liver. In these situations, medication must be discontinued. Cats with concurrent kidney disease will often be placed on a lower dose of thyroid medication. Elevated blood pressure and faster circulation associated with hyperthyroidism result in increased blood flow through the kidney which actually “helps” the kidneys function better. By treating thyroid disease, methimazole is not causing kidney failure, but when it already exists it may appear to worsen with treatment. In severe kidney disease it may not be possible to treat the thyroid disease and monitoring quality of life may be recommended.
Radioactive iodine is an injection that is given subcutaneously (under the skin), similar to a vaccine. The iodine destroys the over-active part of the thyroid gland leaving the normal gland intact. Cats spend a couple of days in hospital while they are excreting the radioactive iodine in their urine. Once released, they follow up for bloodwork after a month to make sure hormone levels are normal. Annual bloodwork is recommended after treatment is concluded. Several screening tests are required prior to radioactive iodine therapy to make sure the cat is a good candidate for treatment. Treatment is costly up front, but over a lifetime can be similar to medication and bloodwork costs.
Untreated thyroid disease will result in continued loss of muscle mass and body weight. Strain is put on the heart and many patients develop high blood pressure and heart disease secondarily.
In a small percentage of cats, the thyroid tumor is malignant. This is often suspected if a cat has a very large thyroid mass (as opposed to a small nodule felt on the thyroid gland) or if they require a very high dose of methimazole to control the disease. Surgery can be done to remove some thyroid tumors. Occasionally supplementation will be necessary following surgery if there is no remaining normal thyroid tissue.
As part of a regular senior wellness program, I recommend having annual bloodwork done. Early recognition of thyroid disease (as well as kidney disease, diabetes, and other common illness of senior pets) will help maximize our ability to treat the disease and preserve quality of life. – Dr. Mary Knott