What is Hyperthyroidism?

The thyroid is a butterfly-shaped gland located around the cat's throat that regulates many aspects of the cat's metabolic rate. The most common hormonal disease of cats, hyperthyroidism is generally caused by the development of benign tumor in the gland. However, a small percentage of the cancers are malignant. It is estimated that hyperthyroidism effects 2% of older cats.

In hyperthyroidism (hyper="high"), too much thyroid hormone is being produced, so your cat's metabolic rate is too high. Hyperthyroidism affects virtually every organ system, and can result in behavioral changes, weight loss, excessive or decreased appetite, hyperactivity or lethargy, fever, rapid or irregular heartbeat, shedding, increased water consumption (polydipsia) and urination (polyuria), vomiting, diarrhea and osteoporosis. In some cats, the disorder produces atypical signs such as depression, inappetance or weakness.

The increase in thyroid hormone causes the cat's heart to beat faster, often > 240 beats per minute. Heart murmers may be present. Heart failure will occur in up to 10% of cats and heart damage occurs in most, although it is usually reversible with treatment of the hyperthyroidism. The haircoat may look scruffy. Enlarged thyroid glands may be found. Effects on the kidneys from the circulatory changes can make existing kidney disease worse or cause the appearance of kidney disease in some cats.

Diabetes, kidney disease, liver disease and other conditions that also affect older cats need to be ruled out prior to settling on a diagnosis of hyperthyroidism, even if tests indicate it is present. In older cats, this disease is common enough that routine screening is considered to be necessary by many veterinarians. Due to the potential for numerous secondary complications, such as heart disease and digestive problems, early diagnosis is a good idea. Consider asking your vet about tests for hyperthyroidism if your cat is over 10 years of age.

Testing for hyperthyroidism is done by measuring the T4 (one thyroid hormone) levels in the blood stream. In most cats with hyperthyroidism, these levels will be above normal. In some cats, they will be in the "normal" range, despite the presence of the disease. This is particularly true of very old cats. In these cats, repeating the test in a week or so is often diagnositic. If not, more specialized testing must be done, such as T3 (another thyroid hormone) suppression tests or thyroid releasing hormone (TRH) testing.

The effects of too much thyroid hormone on the body are so devastating, the condition is fatal if left untreated. An untreated cat will live on average about two years. There are currently three commonly used treatments for this problem. Surgery, radioactive iodine therapy and medical treatment using methimazole (Tapazole ®). Treatment choice will depend on a number of factors, including your cat's general health, finances, and personal preference.

Treatment Options

• Anti-thyroid drugs (Tapazole):
  • Do not cure the disease.
  • Medical treatment may be effective long term if the cat is compliant about taking pills and no side effects occur.
  • Side effects are common and most occur within the first month of treatment. Side effects can include nausea, vomiting, lethargy, lack of appetite, hair loss, skin irritation. With long-term use, effects may include prolonged blood clotting times, liver and kidney damage, and decreased white blood cell counts.
  • Requires pilling your cat 1-3 times daily.
  • Periodic blood tests are required to monitor thyroid hormone levels and potential side effects.
  • Costs vary by region, but are estimated at $400-600/year. Medical treatment is less costly in the short run but over the lifetime of the cat, it may be more expensive than alternative methods of treatment.
  • No surgery risks
    
• Surgery:
  • Surgery is an effective procedure in most cats.
  • Anesthesia risks
  • Possible damage to/removal of parathyroid glands. These glands control calcium regulation in the body and are easily damaged during surgery. Death can result if calcium levels drop sufficiently. Therefore, calcium levels should be carefully monitored for a week if both sides of the thyroid gland are affected.
  • Persistence of hyperthyroidism post-surgery (80% will develop a tumor in the opposite side within one and a half years)
  • Thyroid tissue may lie in the chest making removable difficult or impossible
  • Cost of one surgery is $600-$1200
    
• Radioiodine (I-131)
  • No surgery or anesthesia is required and the parathyroid glands are not affected.
  • An injection of radioactive iodine (I-131) is injected and absorbed only by the thyroid
  • 3% of cats will require a second injection
  • The thyroid function should become normal within one month
  • Few cats require thyroid supplementation following this procedure
  • Your cat must reach the safe and legal level of radiation release (about a week) before coming home

Cat owners report that though tapazol is not cheap, it works very well for many.

Sources:

The 5 minute veterinary consult
Ettingers - the $45 paperback handbook version of the $200 text book.
Plumb's veterinary drug handbook. Gives incidence of side effects for tapazol and other meds, dose schedules, and I think this is where the follow-up blood test schedule is - (if giving tapazol - full blood chem panel monthly for 3 months, then T4 test every 3-4 months).

Useful Web sites :

RadioCat: part sales pitch for radioactive treatment, part information

Thyroid Info