Feline Hyperthyroidism

Feline hyperthyroidism is a common endocrine disorder that occurs when a cat's thyroid produces excess thyroid hormones, mainly T3 and T4.  It usually presents in middle-aged or older cats, and there are no differences among gender or breed in terms of prevalence of the disease.

According to Cornell University, 98% of hyperthyroidism is caused by an adenoma, a non-cancerous tumor on the thyroid gland.  In only two percent of cases, hyperthyroidism is caused by a carcinoma, a cancerous tumor on the thyroid gland.

Because thyroid hormones affect many organs in the body, secondary conditions may occur as a result of feline hyperthyroidism.  For instance, excess thyroid hormones can cause the heart to beat faster.  Over time, cats with hyperthyroidism can develop an enlargement and thickening of the heart's left ventricle.  If left untreated, these changes can affect the normal functioning of the heart and may even lead to heart failure.  Fortunately, once the hyperthyroidism has been treated, the changes to the heart will often improve or resolve completely.

Additionally, cats with hyperthyroidism may also develop hypertension (high blood pressure).  Hypertension can cause damage to the kidneys, eyes, heart, and brain.  If a cat has hypertension in addition to an overactive thyroid, medication may be prescribed to manage the blood pressure.  After the hyperthyroidism has been treated, blood pressure issues will often resolve, and medication to treat hypertension may no longer be necessary.

Symptoms

Because thyroid hormones affect the functioning of many of the organ systems within the body, symptoms of feline hyperthyroidism may vary.  A cat may experience only one or two of the symptoms of hyperthyroidism.  On the other hand, a cat may experience many or all of the symptoms.  Symptoms of feline hyperthyroidism include:

• Weight loss.
• Increased appetite.
• Nervousness or anxiety.
• Increase in activity level or restlessness.
• Increased thirst.
• Increased urination.
• Poor coat condition.
• Diarrhea.
• Muscle weakness.
• Increased respiration rate.
• Difficulty breathing.
• Increased heart rate.

Occasionally, cats with an overactive thyroid may experience decreased appetite or depression as well.  

Signs of the disease may be subtle at first but usually become more prominent as the disease progresses.  

Diagnosis

A diagnosis of hyperthyroidism is typically made when thyroid hormone levels in a cat's blood are high.  However, occasionally cats exhibiting symptoms of hyperthyroidism have normal hormone levels.  In these cases, a test called a T3 suppression test may be performed.  

If a diagnosis cannot be made from blood tests, a nuclear medicine scan may be ordered.  This test is done at certain specialty veterinary clinics.  A small dose of a radioactive compound is given to the cat.  The compound travels through the bloodstream to the thyroid.  Overactive thyroids accumulate more of the compound than do normally functioning thyroids.  After the test is complete, the kitty must remain in the hospital for a few days until the compound has cleared its body.  

Other blood tests and a urinalysis are also usually done to check the functioning of the other organs in the body as well.  

Treatment

There are three options for treating feline hyperthyroidism:

• Medication.
• Surgery.
• Radioactive iodine therapy.

Each treatment option has its own advantages and disadvantages, and a cat's specific circumstances will often dictate which treatment is best for him or her.

Medication: Methimazole (Tapizole) is the medication used to treat feline hyperthyroidism.  It works by reducing the production and release of thyroid hormone from the thyroid gland.  The medication must be given one to three times a day and typically takes several weeks to reduce the hormone levels in the blood.  

Methimazole may have side effects including depression, lack of appetite (anorexia), vomiting, and fever.  These side effects may resolve without having to stop the treatment.  A more serious side effect is anemia.  If this side effect occurs, it typically happens within the first three months of treatment.  If anemia occurs, the medication is usually stopped and another treatment option will need to be considered.

Methimazole is relatively inexpensive and readily available.  Some owners find pilling their cat difficult or even impossible.  In these cases, owners can get the medication compounded into liquid, oral paste, chewable tablets, medi-melts, chewable treats, or transdermal gel.  While getting the medication compounded is more expensive, it's a great option for those who cannot pill their kitties.

Methimazole must be given for life.  If the medication is stopped and another treatment option isn't pursued, a cat's thyroid hormone levels will return to being too high.  Routine blood tests will need to be performed periodically to check a cat's thyroid levels to make sure the medication is effective and to check for potential side effects.

Surgical Removal: Surgical thyroidectomy, removal of the thyroid glands, is a relatively straightforward procedure with good success rates.  The advantage of surgically removing the thyroid glands is that it produces a permanent cure in most cats.  

As with any surgery, though, there are risks to a thyroidectomy.  Damage to the parathyroid glands may occur, for instance.  The parathyroid glands are clse to or within the thyroid glands and are critical to maintaining proper blood calcium levels.  Damage to the parathyroid glands may mean that a cat needs to take regular calcium or vitamin D supplements to maintain proper blood calcium levels.  

Additionally, damage to the nerves near the thyroid glands may occur during surgery.  Nerve damage may result in abnormal pupil size in the eyes and droopy eyelids.  Voice box damage, resulting in a change in a cat's voice, may also occur.  

Some cats who have a thyroidectomy develop hypothyroidism (abnormally low thyroid levels) and must take thyroid pills.  On the other hand, a few cats who have the surgery remain hyperthyroid.  In these cases, there are thyroid cells in abnormal places in the cat's body, such as in the chest, where surgical removal is difficult.  Thyroid tissue in other parts of the body is known as ectopic thyroid.  If an owner and veterinarian determine surgery is the best treatment option for a cat with hyperthyroidism, a nuclear medicine scan may be performed before surgery to check for ectopic thyroid.  If ectopic thyroid is found in the kitty's body, surgery probably isn't the best treatment option.  Additionally, surgery may not be the best treatment option for cats with other conditions such as kidney disease or heart disease.

Radioactive Iodine Therapy: Radioactive iodine therapy is only performed at specialty veterinary clinics.  It is usually very effective and rarely causes cats to develop hypothyroidism.  In radioactive iodine therapy, radioactive iodine is administered to the kitty intravenously.  The radioactive iodine destroys abnormal thyroid cells while leaving normal thyroid cells in tact.  It does not affect any of the cat's other organs.

Radioactive iodine therapy is curative in approximately 95% of hyperthyroid cases and has no side effects.  The procedure does not require anesthesia.  However, it can only be performed at clinics that have a license to utilize radioisotopes.  Radioactive iodine therapy has no significant risks for cats.  However, humans who have close contact with the kitty must take precautionary measures.  A cat must remain in the hospital until his or her radiation level has dropped within acceptable limits.  This means that a kitty receiving radioactive iodine therapy may need to be hospitalized for one to three weeks.

For the few cats who remain hyperthyroid after radioactive iodine therapy, the treatment can be repeated.  In rare cases, cats may become hypothyroid after this type of therapy.  In these cases, thyroid hormone replacement may be necessary.

If you notice any of the symptoms of hyperthyroidism in your cat, please take him or her to the vet for an exam, blood tests, and a urinalysis.  If your kitty has this common endocrine disorder, you and your vet can determine which treatment option would be best for your companion.

Sources:

Cornell University Hospital for Animals: Hyperthyroidism in Cats.

Washington State University College of Veterinary Medicine: Hyperthyroidism in the Cat.

Cornell University College of Veterinary Medicine: Hyperthyroidism in Cats.