Hyperthyroidism

GENERAL DESCRIPTION 

Hyperthyroidism is a common disorder of the endocrine system found mostly in middle aged to older cats. It occurs when there is an excess amount of thyroid hormones produced thus causing an increase in metabolic rate. It can effect multiple body systems and organs including the kidneys, liver, and heart.

SOURCE & CAUSE(S)

The thyroid glands are located in the neck below the voicebox or larynx. Under normal conditions they cannot be felt when you’re petting your cat on the neck. These glands produce two hormones Thyroxine (T4) and Triiodothyronine (T3), and when they are produced in excess, it increases the rate at which the body processes energy, thus increasing the metabolic rate. The production of hormones is controlled by the thyroid stimulating hormone (TSH), which is located at the base of the brain in the pituitary gland. Benign tumors (adenomas) in the thyroid glands is the most common cause behind hyperthyroidism. In 1-2% of cases, the tumor is malignant (cancerous), and the glands will need to be removed as soon as possible.

SIGNS & SYMPTOMS 

• Increased appetite-most hyperthyroid cats will eat constantly, and still lose weight.
• Weight loss-associated with increased appetite
• Excessive drinking and urination
• Hyperactivity
• Vomiting
• Diarrhea
• Rapid heart rate*
• Rapid respiratory rates*
• Heart murmur

Hyperthyroidism effects your pet’s heart, and can lead to rapid heart rate, heart murmurs, and thickening of the heart muscles. This can lead to heart failure and eventually death. Your sick pet will need veterinary attention as soon as possible.

DIAGNOSIS & TESTS

Diagnosis begins with a complete history and a physical exam. There are several different tests used to diagnose hyperthyroidism. The diagnostic path chosen will depend largely on the symptoms your pet has, and the availability of diagnostic tools your veterinarian has. Your veterinarian will be most likely do the following: 

• Palpation of Thyroid Gland - Normally the thyroid gland cannot be “felt”, or palpated, however with hyperthyroidism the gland typically enlarges in size and your veterinarian can palpate this during a physical exam.
• CBC/Chemistry Panel - These blood tests will evaluate various internal organ functions, including the heart, liver, kidneys, pancreas, metabolism, and electrolyte balance. The CBC is a measure the amount and different kinds of red and white blood cells are present in the body. Since hyperthyroidism can cause a disruption of the function of several organ systems in the body, this blood test will measure and indicate if there is any significant dysfunction with the liver, pancreas and kidneys. Cats with hyperthyroidism will generally have a higher red blood cell count.
• Urinalysis  - A urine sample may be collected and the results will reveal the function of your pet’s kidneys.
• Baseline T4 or Total T4 (TT4) - This blood test will determine the level of T4 thyroid hormone in the bloodstream. Cats that are experiencing hyperthyroidism will reveal a high level of T4 in the bloodstream. There are other conditions that can cause an increase of T4 such as kidney or heart disease, so if one of these is suspected after a positive T4 test, additional more definitive tests are run to confirm the diagnosis of hyperthyroidism.
• Baseline T3 Test - This blood test will determine the level of T3 thyroid hormone present in the bloodstream. T3 is another form of thyroid hormone produced by the thyroid gland, it is known as Triiodothyronine. The T3 test is not a very reliable diagnostic tool for testing hyperthyroidism in cats, because in 25% of hyperthyroid cats the T3 levels are not elevated.
• TSH Stimulation Test - This is generally the thought to be the most definitive blood test used for confirming hypothyroidism in dogs. This is a timed test so you should expect to have your dog stay in the hospital for a day. A small amount of Thyroid Stimulating Hormone (TSH) is injected into the bloodstream, then in about six hours a blood sample is drawn for evaluation of the T4 level. If your dog has Hypothyroidism will not have a higher T4 level than that of the baseline value. This is because they are not exhibiting the ability to stimulate the thyroid hormone following and injection the TSH, thus leading the diagnosis of Hypothyroidism. NOTE: Dogs that have high T4 following the TSH test have some other disease process going on that is giving them a low T4 on the baseline T4 test. Your veterinarian will now know that Hypothyroidism is not the cause of the low level T4, and will continue to diagnose.
• Radiographs - This may show an enlarged, thickened heart muscle (hypertrophic cardiomyopathy)
• Cardiac Ultrasound - This may show an enlarged, thickened heart muscle (hypertrophic cardiomyopathy)  

TREATMENT & MANAGEMENT

Because of the high blood pressure associated with this disease, there is an increase in blood flow to the kidneys as well. Treating the hyperthyroidism will decrease blood to the kidneys and could exacerbate any existing kidney disease/failure.

Hyperthyroidism can be treated in various different ways. Your veterinarian will most likely recommend on of the following treatment options available for hyperthyroidism:

Thyroidectomy

Removal of one or both lobes of the thyroid gland can resolve hyperthyroidism and eliminate the need for lifetime medication. It does require going under anesthesia and there are some dangers that come along with it. The major complication of this option is the possible destruction of the parathyroid glands.  These glands are very closely situated to the thyroid; they play a role in regulation of Calcium balance in the body.  Great care must be taken to keep these glands intact, or at least one of them.  The result if losing these glands is having to have your cat take Ca+ and Vitamin D supplements for life.  If a thyroidectomy is performed and is successful, many cats are able to cope without a thyroid glands and don’t need any additional hormonal supplements.  However there are still many cats that post surgically will have to take a thyroid hormone supplement.

Radioactive Iodine Treatment

Treatment with radioactive iodine is a relatively new effective treatment. Because the thyroid gland is the only part of the body that will facilitate the uptake and use of iodine, inserting radioactive iodine can destroy the adenoma in the gland, thus reducing it’s size and hormone production. Your cat will be radioactive for a few weeks, and will need to be kept isolated until it clears.

Long Term Drug Therapy

A very conservative non-surgical treatment involves giving medication that suppresses the overproduction of the thyroid hormone for life. Anti-thyroid drugs such as Methimazole and Felimazole can be prescribed to your cat to block thyroid hormone production. The drug is usually given twice a day over the lifetime of your cat. These drugs do not cure the problem, and the adenoma will continue to grow. However, symptoms can immediately improve after taking these medications. Also, there are side effects associated with these drugs and your cat will need to be routinely checked once or twice a year to make sure their hormone levels are in check and the dosage of medication is still pertinent.

Chemical Ablation

This is a relatively new treatment. Your cat’s thyroid glands are located using an ultrasound and injected with a chemical meant to kill the abnormal cells.

PREVENTION & HELPFUL TIPS

There is no way to prevent hyperthyroidism, but early detection can prevent your pet from displaying severe symptoms.