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The thyroid gland is a butterfly shaped gland that is found just below the larynx (voice box) in front of the trachea (windpipe). This gland is responsible for producing thyroid hormones T3 and T4, which are released into the blood stream secondary to the signal from the brain (hypothalamus and pituitary glands). These hormones are responsible for maintaining the bodies many functions including heart rate, breathing, intestinal movement, attitude, sweat glands and overall metabolism. When this gland becomes over stimulated and excessive thyroid hormone is released the entire body is affected.

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What causes the T3 and T4 levels to increase?

The thyroid levels increase due to a tumor, most often benign (96-97%) that is associated with the thyroid gland. This tumor secretes thyroid hormone and does not respond to the normal signals of the body to stop producing and releasing this hormone in spite of adequate levels.

CLINICAL SIGNS

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What are the clinical signs of hyperthyroidism?

Because the entire body can be affected each hyperthyroid patient may manifest their disease differently. The most common signs of hyperthyroidism include:

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Increased heart rate or tachyarrhythmias.

A very common and frequently over looked clinical sign is tachycardia. Since so many cats are stressed in the office veterinarians may assume it is benign or expected. It can be from thyroid toxic effects directly on the rate centers of the heart and also from tachycardia associated with heart failure from cardiac muscle changes due to thyrotoxic (excess thyroid hormone) effects.

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Weight loss

An obvious clinical sign but, many times it is not noticed until the patient has lost more then 20% of its original body mass. This is the most common presenting complaint to the veterinarian.

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Change in appetite

Although not always obvious to the owner, increases in the cat’s appetite are early signs of hyperthyroidism. Increasing feeding frequency and amount is a common signs. A smaller percentage of cats will show a loss of appetite. This is noted when other organ systems are being affected.

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Change in behavior (more hyperactive/ lethargic)

Increased activity, lack of normal sleep habits, playing more aggressively, more irritable then normal, pacing around the house, wide eyed look; are common early signs of hyperthyroidism. However, do not over look the patient that is lethargic and sleeping all the time as organs systems begin to fail, systemic illness can override the typical hyperactive patient.

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Vomiting

Although feline patients do have a number of reasons for vomiting, retrospective studies have shown that cats with hyperthyroidism, have a clinical presentation of vomiting frequently but are initially evaluated for other gastro-intestinal diseases.

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Diarrhea

Primary gastro-intestinal disease such as inflammatory bowel disease (IBD), Lymphoma, bacterial over growth, parasitism and food allergies are all common causes of diarrhea. However, many hyperthyroid patients have diarrhea with none of the above mentioned conditions.

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Hair loss

Hair coat changes such as dry hair, fragile, thinning, or even outright patches of alopecia are some of the main ways hyperthyroid cats present with dermatological signs. Older cats with recent changes in skin and hair coat should be screened for hyperthyroidism.

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Increased Respiratory Rate

Subtle increased rates of respiration with no stress or physical excretion can be due to excess thyroid hormone or even thyrotoxic heart failure.

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Fever

Due to the thyrotoxic affects systemically some patients present with a low to moderate grade fever with no evidence of sepsis or necrosis.

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Vocalization changes

Owners frequently complain that their pet is either vocalizing more or has changed their tone or sound of their meows or cries. This can be very pronounced especially at night.

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TESTING

I think my cat is hyperthyroid, what tests should be done?

The screening tests for hyperthyroidism include a baseline total thyroid hormone level (TT4) and a free thyroid hormone evaluation by equilibrium dialysis (free T4 by equilibrium dialysis).

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What if the screening tests are inconclusive, what other test are available to evaluate my cat’s thyroid status?

If these tests are inconclusive a T3 suppression test is recommended. This test further evaluates the pituitary thyroid axis and requires initial blood work in concert with administration of thyroid hormone for 3 days and repeat blood tests taken after medication administration.

Other tests that are utilized include the TRH stimulation test and the TSH test.

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What about thyroid scintigraphy, is it necessary for appropriate diagnosis and treatment of hyperthyroidism?

No. However, thyroid scintigraphy can be helpful for unclear or occult cases of hyperthyroidism, but is typically not necessary for definitive diagnosis or more importantly treatment of the disease.

TREATMENT

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My cat was determined to be hyperthyroid, now what?

There are three most common treatment options; here are the advantages and disadvantages of each:

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Tapazole

Advantages

Oral medication

Cheaper short term option

Short term and long term option

Disadvantages

Oral medication

Side effects seen in 20-25% of cats include lethargy, liver disease, vomiting and diarrhea, facial itching and scabbing, bone marrow suppression

Expensive long term

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Surgery

Advantages

Long term option

Disadvantages

Increased upfront cost

Surgical side effects including hypocalcemia (low calcium), laryngeal paralysis and anesthetic risk

Ectopic tissue (see diagnosis page)

Hospitalization

Often need more than one surgery with recurrent disease

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Radioactive Iodine Therapy

Advantages

Effective cure with one subcutaneous injection in 93-95% of patients

Minimal side effects

Cheapest long term treatment

Disadvantages

Hospitalization (generally 2-3 days)

Upfront cost

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Are other treatment options available?

Iopanic Acid

An iodinated contrast material has been used orally to suppress the conversion of T4 into T3. This option worked best in mild-moderate cases versus those with severe long standing disease. This medication needs to be compounded for use. No long term studies are currently available for this medication.

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Are there any contraindications to treating hyperthyroidism?

The most significant concern is unmasking underlying renal disease, which although unlikely is a potential side effect with ANY treatment option.

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Why is renal failure potentially worsened by treating hyperthyroidism?

Blood flow to the kidneys is increased in hyperthyroid patients and this promotes the function of the kidneys. Because renal insufficiency is seen in older felines the increase in blood flow secondary to the hyperthyroidism essentially improves the function at least for the short term. When these patients are treated for their hyperthyroidism and the blood flow to the kidneys decreases or normalizes, the function of the kidney worsens. This if often reflected in an increase in both blood urea nitrogen (BUN) and creatinine and a decrease in urine specific gravity. Generally this decrease in function is not clinically a problem, however, a small percentage of cats become clinical for their renal disease.

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Can renal disease be prevented?

Although not totally preventable, there are some precautions that can be taken prior to treatment that will decrease the incidence of renal disease. This includes a thorough evaluation of the kidneys including blood work and a urinalysis. If there are any concerns regarding kidney function a tapazole trial at 2.5 mg orally twice a day for 2 weeks with a repeat of lab work including a renal profile, T4 level and urinalysis are recommended. If there is any worsening of the kidneys then a long term treatment option like iodine therapy or surgery may not be the best option. If the kidneys do not worsen but the thyroid level is still high, increase the tapazole to 5 mg orally twice a day and repeat the lab tests as before. If the kidneys improve or do not worsen then a long term treatment option is possible.

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What if my cat shows renal signs prior to treatment?

Some hyperthyroid patients with renal disease can be placed on a low dose of tapazole that may help with the clinical signs, but not totally control the hyperthyroidism, while not worsening the renal disease. Sometimes it is better to achieve a balance in a patient in renal disease with some degree of hyperthyroidism.

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Are there any precautions that I must take after my cat has radioactive iodine therapy?

We will give you detailed instructions at the time of discharge; however we will have you use clumpable flushable litter to dispose of the urine and feces. Any person in the household under 18, pregnant or trying to get pregnant should limit their interaction with the cat for the first 2 weeks (especially the urine and feces). The precautions are common sense but are also mandated by the Nuclear Regulatory Commission.

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Are there any post-treatment concerns with radioactive iodine therapy?

Other than the concern for the kidneys, other potential side effects include mild dysphagia, transient lethargy, and decreased appetite.

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The post treatment thyroid level was low to low normal, do I need to give thyroid supplementation?

It is not unusual for the initial thyroid levels to be low. It can take some time for the atrophied normal tissue to recover and produce thyroid hormone. If the thyroid level is low AND the patient is lethargic or inappetent, thyroid hormone supplementation may be necessary. Approximately 10% of iodine treated patients need prolonged thyroid hormone replacement.