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Hyperthyroidism in Cats

By: Dr. Michael Bernstein

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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 The T3 Suppression test has largely been replaced by the Serum Free T4 by Equilibrium Dialysis (FT4ED) test.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.

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