Thyroid Disoders: Hyperthyroidism in Cats and Hypo

Bedford Highway Veterinary Hospital

936 Bedford Highway
Bedford, NS B4A 3P1


What is the thyroid?

 The thyroid gland produces hormones which are vital to your pet's metabolism. When it is underactive, the pet's metabolism slows down, and when it is overactive, your pet's metabolism speeds up to unhealthy levels. Both conditions are relatively easy to control using affordable medications. Hyperthyroidism (overactive thyroid) in cats and hypothyroidism (underactive thyroid) in dogs are relatively common disorders. Strangely enough, dogs almost never develop an overactive thyroid while cats almost never develop an underactive thyroid.

What causes hyperthyroidism in cats?

The most common cause of hyperthyroidism is a tumour, usually benign, on the thyroid which results in increased production of the thyroid hormones.

What are the symptoms of hyperthyroidism in cats?

Many of the symptoms of hyperthyroidism mimic other diseases, such as diabetes or kidney failure. You may see generalized symptoms such as weight loss, increased thirst, increased appetite, vomiting, increased vocalizing, and restlessness. Because hyperthyroidism is so common, and because its symptoms are similar to many other diseases, we recommend screening your cat for hyperthyroidism regularly during the senior years.

How is hyperthyroidism diagnosed?

In cats, a blood test checks the level of T4 (one of the thyroid hormones) in the blood. If your cat is hyperthyroid, the levels of T4 in the blood will be higher than normal.

How is hyperthyroidism treated?

The most common treatment is through methimazole therapy. Methimazole (brand name Tapazole) is an antithyroid drug which can be given orally, or through the skin in a gel form. After starting your cat on methimizole therapy, we will want to recheck the T4 level in the blood within a few weeks to make sure that we are giving the right dose.  If it turns out that we are giving too much, or not enough, this may need to be repeated multiple times until we get the dose just right. Soon you will see your pet looking calmer, sleeker, and will begin to regain the weight he/she lost. Once we get your pet balanced on a healthy dose, we will still want to recheck that T4 level every 6 months to a year to make sure that the balance is being maintained. Legally, we have to recheck T4 levels at least once a year before we can refill prescriptions. If you notice that your cat is showing a return of symptoms, we recommend rechecking his/her T4, whether or not he/she is officially "due" for a recheck.

What causes hypothyroidism in dogs?

Most cases of hypothyroidism in dogs are a result of the dog's own immune system attacking thyroid tissues, but there may be other causes involved in your own pet's case.

What are the symptoms of hypothyroidism?

Symptoms may include weight gain despite a small appetite, hair loss, lethargy, dry skin, skin or ear infections, and getting cold easily.

How is hypothyroidism diagnosed?

With dogs, we recommend a full thyroid profile blood test, which measures T3 and T4 (thyroid hormones) as well as levels of TSH (thyroid stimulating hormone, an endocrine produced by the pituitary gland). Once we have diagnosed your pet, it is usually only necessary to check one of the thyroid hormones, T4, to monitor how well the medications are working.

How is hypothyroidism treated?

Hypothyroidism is treated by giving a synthetic thyroid hormone to boost your dog's metabolism back to normal levels. After starting medication, we will recheck your pet's thyroid by performing a T4 blood test. This test will tell us if the medication has put your pet's thyroid levels back into normal range. We may need to change the dose several times before finding the right balance. Your will notice that your dog is losing weight, more active, and generally happier. One your pet is successfully maintained at normal levels, we will only require a blood test every 6 months to a year to make sure that the medication is still doing its job. If you notice your dog regaining lost weight, losing fur again, or any other symptoms beginning to return, we recommend rechecking his/her thyroid levels at that time.