Several other diseases, including chronic renal failure, liver disease, and cancer, can have similar symptoms as hyperthyroidism. A thorough history, physical exam, and routine laboratory testing will lead to the diagnosis of hyperthyroidism in the majority of cases. Routine testing includes a complete blood count, or CBC, a blood chemistry profile, urinalysis, and a test of the thyroid hormone concentration called a serum T4.
Cats with early signs of hyperthyroidism initially may show a thyroid hormone, or T4, level that is within the normal range, making diagnosis more difficult. Because hormone levels may fluctuate between normal and increased early in the course of the disease, it is important that cats showing early signs have their thyroid hormone levels evaluated after a period of one to two weeks. Sometimes more extensive thyroid tests, such as a free T4 or T3 suppression test, a thyrotropin-releasing hormone stimulation test, or a radionuclide thyroid scan may be required. Veterinarians in general practice may refer cats to a veterinary internist or veterinary teaching hospital for some of these procedures.
