What is hyperthyroidism?

Hyperthyroidism is an endocrine disease caused by excess production of thyroid hormone.

What causes hyperthyroidism?

The majority of cases of feline hyperthyroidism are caused by a benign tumor (adenoma) of the thyroid gland. One or (more commonly) both lobes of the gland may be involved. Rarely, this tumor is malignant (adenocarcinoma).

What are the signs of hyperthyroidism?

Hyperthyroid cats present in a variety of ways. They may exhibit any or all of the following signs:

1. Weight loss despite increased appetite
2. Unkempt hair coat (matted or patchy hair, excessive shedding)
3. Increased drinking and urination
4. Hyperactivity (increased vocalization, restlessness)
5. Vomiting and/or diarrhea
6. Heat intolerance (seeking cooler areas)
7. Cardiovascular abnormalities (increased heart rate, irregular heart rhythm, heart murmur, high blood pressure, heart disease)
8. Respiratory abnormalities (panting, respiratory distress)

How is hyperthyroidism diagnosed?

Physical exam: including evaluation of your cat’s overall appearance, listening to the heart and lungs, and palpating the thyroid gland.

1. Blood work and urinalysis
   • Thyroid level (total T₄):  The total T₄ level includes thyroid hormone bound to protein as well as unbound hormone.  If sufficiently elevated, a diagnosis of hyperthyroidism can be made. However, in some cases a cat that is suspected of being hyperthyroid may have a normal T₄ level. This may be the case early on in the disease or if your cat has concurrent illness. If hyperthyroidism is suspected despite a normal T₄ result, additional diagnostics such as a free T₄ (fT₄) level can be done.  The free T₄ test measures thyroid hormone that is not bound to protein in the bloodstream.
   • Liver enzymes: Hyperthyroid cats often have elevated liver enzymes.
   • Kidney enzymes: Many hyperthyroid cats have concurrent kidney disease.  In these cases, the kidney disease becomes more evident when the hyperthyroidism is treated.
   • Urine specific gravity: Another indicator of kidney function.

Cardiac workup: If a heart murmur or arrhythmia is ausculted, an echocardiogram, ECG (electrocardiogram), blood pressure and chest x-rays may be recommended.

How is hyperthyroidism treated?

There are four main treatments for hyperthyroidism.

1. Medication

Methimazole (Tapazole®) is a drug used to inhibit thyroid hormone synthesis. It is available as an oral tablet or compounded liquid or transdermal gel applied to the ear. Methimazole must be given twice a day for the lifetime of your cat.

After you cat is started on methimazole, you will need to bring the cat back for a recheck appointment every 3-4 weeks until the thyroid level is well regulated. After that, your veterinarian will schedule rechecks every 3-6 months depending upon other factors such as concurrent kidney disease. Over time your cat may need the medication amounts adjusted as their hormone levels fluctuate.

Possible side effects of methimazole include decreased appetite, vomiting and lethargy. These side effects usually resolve with time and do not require stopping the medication but we may need to decrease the dose. Very rarely methimazole can cause liver toxicity, white blood cell suppression and scratching of the face. If these occur, the medication is stopped and we would try a different treatment.

Methimazole is the treatment of choice for cats with concurrent kidney disease because it can be titrated to an appropriate dose to control hyperthyroidism while managing elevations in kidney values.

2. Radioactive Iodine

This treatment can cure hyperthyroidism with a single injection (in most cases) of radioactive Iodine 131. The radiation targets and destroys only the abnormal thyroid tissue.

Radioactive iodine therapy is performed only at selected facilities including the University of Pennsylvania School of Veterinary Medicine and RadioCat, as well as some area specialty hospitals. Cats are required to stay in the designated facility for a few days following treatment until a safe radiation level is achieved.

Post-treatment monitoring includes checking the thyroid level and kidney values one month after treatment and every 6-12 months thereafter.

The cost associated with radioactive iodine is over $1,200 but in some cases this may end up being cheaper over the life time of your cat.

3. Surgery

Removal of the thyroid gland may also cure hyperthyroidism. However, there are several potential complications associated with thyroidectomy. Surgery requires general anesthesia, which may carry an increased risk in cats with heart or kidney disease. The parathyroid glands, which are closely associated with the thyroid, may be inadvertently damaged or removed during surgery resulting in low calcium levels. In some cases the cat may become hypothyroid (not enough thyroid hormone) and require thyroid supplementation.

4. Diet

Science Diet Prescription Diet Y/D is a diet low in iodine content. Monitoring is similar to giving methimazole. We check thyroid and kidney levels 4 and 8 weeks after starting the diet, as well as every 3-6 months ongoing for the life of your cat. Diet control is best done in single cat households, but can be in a multi-cat home with consultation from your veterinarian. While eating Y/D your cat cannot eat any other food, treats or flavored medications. Anything you give to your cat should be cleared by your vet.

What is the prognosis for cats with hyperthyroidism?

Overall, the prognosis is good. However, factors such as type of treatment chosen and presence of other disease will affect your cat’s outcome.