Hyperthyroidism in Cats

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Diagnosis In-depth

Diagnostic tests are needed to recognize feline hyperthyroidism and exclude all other diseases. Diagnostics should include a complete medical history and physical examination. Many cats with hyperthyroidism are old and may have concurrent medical problems that can affect the choice of therapy. Your veterinarian will most likely recommend the following diagnostic tests:

  • A complete blood count (CBC) to evaluate the patient’s red and white blood cell counts. It’s important to know these values prior to starting therapy, as the medical therapy (Methimazole) most commonly used to treat hyperthyroidism can, in rare instances, adversely affect the cat’s red cell or white blood cell count.
  • A serum chemistry profile is needed to evaluate multiple organ systems, including the liver and kidneys. It’s important to know the status of all body systems prior to instituting therapy for hyperthyroidism and to identify any concurrent condition that may complicate therapy.
  • A serum thyroid hormone (T4) level should be evaluated. The serum T4 (thyroxine) measures the level of circulating thyroid hormone in the bloodstream. This is the test most commonly used to determine whether the thyroid gland is hyperactive. The T4 is also monitored once therapy is instituted to determine whether the therapy is effective. Mild high normal elevation with minimal clinical signs may indicate repeating of the thyroid T4 in a few weeks.
  • Serum Free T4 by Equilibrium Dialysis (FT4ED) is used in cats with clinical signs of hyperthyroidism but a normal or high normal value. Elevation of the FT4ED in combination with a high normal T4 is considered diagnostic.
  • Chest X-rays should be taken to evaluate the heart and lungs. Heart enlargement can be associated with hyperthyroidism; if cardiac changes are identified on X-rays, further evaluation of the heart is indicated.

    Additional diagnostic tests may be recommended on an individual pet basis, including:

  • A T3 suppression test. This is a blood test that may be performed to evaluate a patient suspected of being hyperthyroid, but consistently has serum T4 levels that are within the normal range. The T3 Suppression test has largely been replaced by the Serum Free T4 by Equilibrium Dialysis (FT4ED) test.
  • A Technetium-99m scan to evaluate a cat suspected of being hyperthyroid, but with normal serum T4 levels. It is also used to determine if one or both glands are involved there. This test uses a short-acting radioactive isotope to identify abnormal, hyperactive thyroid tissue via a nuclear scan. This test has limited availability and it can be done only in those facilities licensed to handle radioactive materials. Accordingly, it is often a referral procedure.
  • Treatment In-depth

    Treatment for feline hyperthyroidism may include one or more of the following:

  • Medical therapy with Tapazole® (Commonly known as Methimazole). Methimazole is a drug that interferes with the production of thyroid hormone by the thyroid gland; it does not affect the actual physical structure of the thyroid nodule itself. There are other related drugs available in Europe that can be used for the same purpose.
  • Patients receiving Methimazole must have their serum thyroid hormone levels measured periodically. They must also be monitored for any adverse reactions to the Methimazole; these reactions can include drug- induced anemia, low white blood cell counts and liver injury.
  • Treatment with Methimazole must continue for the remainder of the cat’s life and the dosage may have to be adjusted periodically. Most cats tolerate therapy with Methimazole very well.
  • This therapy is also appropriate for cats with heart failure due to hyperthyroidism. This treatment, along with heart failure medication, controls the thyroid condition while the heart failure is “stabilized.”

    Therapy may include surgical removal of the abnormal thyroid lobes. For example:

  • For cats that don’t respond well to medical therapy, or when owners prefer not to medicate their cats on a daily basis, thyroidectomy (removal of the thyroid gland) is a viable alternative. The advantage of this approach is that surgical removal of the abnormal thyroid lobe or lobes is curative. However, since approximately 75 percent of hyperthyroid cats have adenomas involving both thyroid lobes, a bilateral thyroidectomy is required to control the hyperthyroidism.
  • Following bilateral thyroidectomy, daily supplementation with thyroid hormone is required.
  • In addition, there is a risk of damaging or removing all four of the parathyroid glands, which regulate calcium levels in the cat’s body and are adjacent to the thyroid gland. Damage to, or removal of, the parathyroid glands would result in serious medical consequences, requiring careful monitoring and therapeutic intervention.

    Therapy may include radioactive iodine treatment. For example:

  • Radioactive iodine therapy (I131) is another potential treatment for hyperthyroidism. The advantage of this therapy is that it is usually a one-time treatment, does not subject the cat to the risk of anesthesia and surgery, has no need for long-term medication and follow-up care is usually not required.
  • The disadvantage of this form of treatment is that it is available only at facilities that are licensed to handle radioactive isotopes, and hospitalization of the patient for a minimum of 7 to 10 days is usually required in most states. Radiation safety laws in each individual state dictate length of hospitalization; these laws require that the patient be kept in isolation until the radioactivity from the isotope that was administered has fallen to a certain level.
  • Another disadvantage of this form of therapy is cost; however, although the initial cost may be significantly more than medical therapy, over time it is a very cost-effective means of treatment since, after successful therapy, there is no need for daily long-term medication or frequent visits to the veterinarian to monitor the patient.

    Other medical treatments may include:

  • A new treatment that involves ultrasound-guided delivery of alcohol into the enlarged thyroid gland to kill the tissue has been reported, but requires further refinement to demonstrate safety.
  • Beta-blockers (atenolol) are sometimes used for medical therapy when a cat cannot tolerate Methimazole treatment. This therapy blocks some of the effects of thyroid hormone excess, but it is not an ideal treatment.
  • Dietary therapy with Hill’s Prescription Diet y/d may be an option in some cats. The diet is low in iodine and may improve thyroid health when fed as the sole food. It may be recommended in pets with mild disease or in conjunction with other treatment methods. Hill’s research suggests that y/d may improve thyroid health in 3 weeks.
  • Concurrent congestive heart failure is sometimes found and requires separate appropriate treatment.
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