Cats unlike dogs as they age can develop elevated thyroid levels. This disease is called hyperthyroidism. In contrast, dogs typically develop and immune-mediated HYPO-thyroidism where they cannot make enough thyroid hormone.
In cats, the cause is usually a benign thyroid tumor. In some rare instances (very small percent) the thyroid tumors are not benign.
The tumor produces excess thyroid hormone which acts to speed up the heart, increase blood pressure, increase metabolism (ravenous appetite), faster gastro-intestinal transit time and increased urination.
The symptoms cat owners typically see is a restless cat that vomits more than usual, an increased appetite, increased drinking and urination. The cat may be more hyperactive and vocal and grab food from the table that they never did before.
On examination the hyperthyroid patient is often:
- underweight
- fast heartbeat, sometimes with a murmur
- occasionally overgroomed fur
- difficulty sitting still
- willingly devouring treats during examination
Most cats with this disease are eights years and older. Based on what is reported at home and the physical exam, bloodwork is done to evaluate organ function and look at the thyroid production.
Most middle aged and older cats don’t have a total T4 (a common feline thyroid measurement) between 1.2 and 2.6. Veterinary suspicion is increased with T4 levels over 3.0, especially if the symptoms fit.
There may be other test needed to make the diagnosis is less obvious cases but once it it determined that a feline patient is hyperthyroid, we start with a medication called methimazole which blocks the step that adds iodine to the hormone synthesis. This way, the active hormone is reduced.
There are many more things involved as we lower a patient’s thyroid production that have to be monitored but for the purpose of this article, we will focus on the thyroid.
Management:
Once an appropriate T4 level is established and the patient’s symptoms have resolved a decision can be made on how to manage the cat’s hyperthyroidism. Medication is one way, but there is a growing tumor even if benign. Another long-term solution is radioactive iodine
Methimazole is the starting medication
Radioactive iodine (I-131). Iodine needed for hormone synthesis. With radioactive iodine treatment the iodine has had a radioactive substance added to it. This is injected into the cat.
The tumor portion of the thyroid, which is overactive, preferentially takes up the iodine so the majority goes directly to the site of the tumor. There the radioactive substance destroys the abnormal tissue, leaving the patient with hopefully normal thyroid tissue behind and eliminating the disease
In some cases a second dose is needed to destroy the abnormal thyroid tumor. In other instances too much iodine is used and too much tissue is destroyed. In this latter case, we still have a stagnant level to work with and we can supplement the patient with thyroid hormone and have a good outcome and no widely variable fluctuations. This is still an appropriate outcome that is easy to manage.
Best case scenario is the dose of radioactive iodine is just right and the patient no longer requires medication and the disease is gone.
There is a lot more that can go along with unmanaged feline hyperthyroidism including hypertension, cardiac issues, kidney failure and more. As you cat is stabilized whether by medication or ultimately radioactive iodine, some monitoring will be needed to ensure we have the right level.
The positive side of this illness, is it can be managed and your cat can have a happy normal life once under control. Talk to your veterinarian if you think your cat may be showing some of the symptoms discussed here.
