Symptoms and Diagnosis of Feline Hyperthyroidism: Key I

Hyperthyroidism in cats often presents with a trio of telltale signs: weight loss, an insatiable appetite, and hyperactivity. However, there are other, less frequently reported symptoms that are still worth mentioning. These include increased urination, thirst, regurgitation, vomiting, diarrhea, an increase in fecal output, and an elevated or labored breathing rate.

The surge in thyroid hormone levels accelerates energy metabolism and heat production in almost all body tissues, leading to an increase in appetite, weight loss, muscle atrophy, and a rise in body temperature.

This hormonal increase also interacts with the nervous system, causing overexcitement, tension, and muscle tremors. Rapid, excessive eating can trigger nausea, a common issue in multicat households. Vomiting may also be a direct effect of elevated thyroid hormone levels in the chemoreceptor trigger zone, a brain area responsible for triggering vomiting.

The increased intestinal motility caused by hyperthyroidism can lead to more frequent diarrhea. The exact reason for the polyuria and polydipsia seen in hyperthyroid cats is not entirely clear. Some cats may also suffer from chronic kidney failure, a condition more common in older felines. Changes in kidney physiology, due to increased blood flow, can prevent the kidneys from concentrating urine properly, leading to more dilute urine and an increased need for water intake.

Longterm elevation of circulating thyroid hormone levels can lead to changes in the heart, known as thyrotoxic cardiomyopathy. Cats with this condition may exhibit symptoms of heart failure, such as rapid breathing or difficulty breathing.

InDepth Diagnosis

Diagnosis involves identifying hyperthyroidism in cats and ruling out all other diseases. This process should include a thorough understanding of the cat's medical history and a comprehensive physical examination. Many cats with hyperthyroidism are elderly and may have other health issues, which can complicate treatment decisions. Your veterinarian may recommend the following diagnostic tests:

1. A complete blood count (CBC) to assess the cat's red and white blood cell counts. It's crucial to know these values before starting treatment, as the most commonly used medication for hyperthyroidism (methimazole) can, in rare cases, negatively affect a cat's red or white blood cell counts.

2. Serum chemistry analysis to evaluate multiple organ systems, including the liver and kidneys. Understanding the status of all body systems and identifying any concurrent conditions that could complicate treatment is important before beginning treatment for hyperthyroidism.

3. Assessment of serum thyroid hormone (T4) levels. This is the most common test used to check for hyperthyroidism. T4 levels are also monitored again after starting treatment to determine the effectiveness of the therapy.

4. The serum free T4 by equilibrium dialysis (FT4ED) method, which can be used to diagnose cats with clinical signs of hyperthyroidism but normal or slightly elevated T4 levels. Elevated FT4ED levels, combined with normal high T4 results, can confirm the diagnosis.

5. Chest Xrays to evaluate the heart and lung conditions. Enlargement of the heart may be associated with hyperthyroidism, and if heart changes are detected on Xray, further cardiac evaluation is necessary.

Other diagnostic tests may be recommended for individual pets, including:

1. The T3 suppression test. This is a blood test for patients suspected of having hyperthyroidism but whose serum T4 levels remain within the normal range. However, the T3 suppression test has been largely replaced by the equilibrium dialysis (FT4ED) test.

2. Technetium99m scans to evaluate cats suspected of having hyperthyroidism but with normal serum T4 levels. It can also be used to determine if one or both glands are involved in the disease process. This test uses a shortlived radioactive isotope to identify abnormally active thyroid tissue through nuclear scanning. However, the availability of this test is limited and can only be performed in facilities approved to handle radioactive materials, making it often a referral procedure.