What is Hyperthyroidism?

Thyroid hormones are hormones produced by the thyroid gland and control the body’s metabolic rate. Hyperthyroidism, or an overactive thyroid gland, is caused by a benign growth in the thyroid gland of cats that leads to overproduction of thyroid hormones. This overproduction of thyroid hormone leads to a hypermetabolic state in our affected cats. Hyperthyroidism is generally a disease of older cats, with the average cat being greater than 13 years old.

What are the symptoms of hyperthyroidism?

The most common sign of hyperthyroidism in our cats is weight loss, despite a good appetite. Hyperthyroidism can also lead to chronic or intermittent vomiting/diarrhea, increased thirst/urination, excessive vocalizing, poor grooming habits and urination outside the litterbox. Many cats will also have an elevated heart rate and sometimes an abnormal heart rhythm or a heart murmur, secondary to the increased metabolism of the heart. Some cats also develop elevated liver values seen on bloodwork, most commonly the ALT. This value tends to resolve with treatment.

How is it diagnosed?

A diagnosis is based based on bloodwork indicating elevated thyroid hormone levels. There are some patients where the total thyroid hormone is borderline and further testing is required. Sometimes the thyroid levels can be in the normal reference range and a cat can still be hyperthyroid. If a pet is suspected to be hyperthyroid and the total thyroid hormone is not elevated, an additional test called a free T4 is recommended. If this test is high, the cat is considered hyperthyroid. Most senior blood panels include a T4 and a Free T4 for our pets.

What is the treatment?

There are a variety of treatment options for hyperthyroid cats including diet change, radiotherapy and oral medications.

  • Radiotherapy - This is generally considered the safest and most effective method of treatment. A cat is given an injection of radioactive iodine (iodine 131) under the skin (similar to a vaccine). The radioactive iodine specifically targets and damages the thyroid tissue so the thyroid gland stops producing excessive thyroid hormones. This treatment does involve staying in a special referral facility until radiation levels have dropped and specific home care afterwards to prevent contamination to humans. Full blood work and chest x-rays are generally recommended prior to treatment.
    • Advantages: Treatment occurs only once and ongoing therapy isn’t required (2-4% require a second treatment). The disease is cured instead of just managed. The injection is not stressful to the pet.
    • Disadvantages: The patient is boarded at a special facility for a couple of days post treatment. There is a special protocol for a couple of weeks once the patient returns home. This is a more expensive therapy. Radiotherapy is not recommended for cats with kidney disease.
  • Oral medication - Oral medication with methimazole or felimazole is the most common mode of treatment. These medications block production of thyroid hormones. Oral medication takes about 4 weeks to be effective.
    • Advantages: The medication is inexpensive. Medication is as effective as radiotherapy once the proper dose is achieved. Treatment can be discontinued if needed and the side effects are reversible. No hospitalization is required. Side effects are uncommon. If a dose is missed, there are no repercussions. If there are no side effects in the first 3 months of treatment, side effects are highly unlikely.
    • Disadvantages: Medication must be given daily to twice a day, 15% of cats experience side effects (lethargy, loss of appetite, vomiting or extreme pruritus). Blood monitoring is required. Gloves are needed to handle medication.
    • *If a cat is unwilling to accept daily pills, this medication can be made into a liquid form through a compounding pharmacy. This medication can also be made into a transdermal gel that is applied to the cat’s ear and absorbed through the skin.
  • Diet - Iodine is necessary to produce thyroid hormones, therefore, by formulating a diet that is restricted in iodine, thyroid hormone levels may be reduced without producing an iodine deficiency. There is one diet formulated by Hills called Y/D and is available in wet and dry. This diet has not been studied in healthy, normal thyroid cats, therefore, for multi-cat homes the normal cat should be supplemented with other food. It is also only effective if a cat is given this diet and this diet only, which means NO OTHER TREATS OR FOOD. This diet takes about 8-12 weeks to be effective.

Treatment of hyperthyroidism may unmask kidney disease. The increased blood flow to the kidneys in hyperthyroid cats can prevent cats with underlying kidney disease from having elevated kidney values, once that metabolism is slowed, kidney disease can be unmasked. In some cats it is necessary to find a happy medium between treating the thyroid and the kidneys. Approximately 15-22% of cats treated for hyperthyroidism will show kidney disease that was not evident prior to treatment.

How is hyperthyroidism monitored long-term?

Routine monitoring of the thyroid hormone and kidney function is generally recommended. It is not uncommon to need to adjust the dose of oral medications in the beginning. Once a stable dose is reached, thyroid testing is recommended every 6 months. A blood pressure measurement is often also recommended at the start of treatment. Untreated hyperthyroidism can lead to an elevated blood pressure and cardiac disease.

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