Hyperthyroidism is a common endocrine disorder in aging cats, often requiring accurate diagnosis for effective management. Thyroid hormone levels are influenced by various factors, including age, nutrition, underlying diseases, and medications. For a comprehensive assessment, especially in cats with multiple health issues, a Feline Thyroid Panel is highly recommended. This panel typically includes Total T4, Total T3, Free T4 by equilibrium dialysis, and TSH.
Key Thyroid Tests for Cats
Several tests are crucial in diagnosing hyperthyroidism in cats. Each test provides unique insights into thyroid function, aiding veterinarians in accurate diagnosis and treatment planning.
Total T4 (Thyroxine)
The Total T4 (T4) test is a primary diagnostic tool for hyperthyroidism. It measures the total thyroxine hormone in the bloodstream. Elevated T4 levels are a strong indicator of hyperthyroidism. This test is also useful for screening for hypothyroidism and monitoring the effectiveness of methimazole treatment. However, it’s important to note that T4 concentrations can be influenced by medications, other diseases, and nutritional status.
Total T3 (Triiodothyronine)
Total T3 (T3) measures the total triiodothyronine, the active form of thyroid hormone. In hyperthyroid cats, T3 levels are usually elevated or at the high end of the normal range. Measuring T3 alongside T4 enhances diagnostic accuracy, especially in cases where T4 levels are borderline.
Free T4 by Equilibrium Dialysis (FT4 ED)
Free T4 (FT4) by equilibrium dialysis (ED) measures the unbound thyroxine, the biologically active portion of T4, in the blood. This test is particularly helpful in diagnosing hyperthyroidism in cats with borderline or high-normal total T4 levels. The equilibrium dialysis (ED) method is considered the gold standard for FT4 measurement in cats. It’s crucial to handle samples properly as FT4 ED levels can be falsely elevated by mishandling, such as warming or severe hemolysis. Low protein levels due to kidney or liver disease can also affect FT4 concentrations. The Free T4 Two Step (TS) method, valid for canine samples, is not appropriate for feline samples.
TSH Baseline (Thyroid Stimulating Hormone)
Thyroid Stimulating Hormone (TSH) measurement complements other thyroid tests. In hyperthyroid cats, TSH levels are typically low due to negative feedback from elevated thyroid hormones on the pituitary gland. Conversely, cats showing signs of hypothyroidism after radioactive iodine treatment for hyperthyroidism will generally exhibit high TSH levels. Measuring TSH in conjunction with T4 and FT4 provides a more complete picture of thyroid function.
T3-Suppression Test
The T3-suppression test is used in specific cases to diagnose hyperthyroidism in cats that exhibit clinical signs of the condition but have normal baseline T3, T4, and/or free T4 concentrations. This test assesses the thyroid gland’s response to T3 administration.
Procedure for T3-Suppression Test:
- Collect an initial (baseline) blood sample and freeze it.
- Administer 25 μg of T3 orally (e.g., Cytobin®) every 8 hours for 2 days, with a seventh dose on the morning of the third day.
- Collect a second blood sample 4 hours after the final T3 dose. Process this sample similarly to the baseline sample.
- Submit both paired serum samples together, ensuring they are shipped chilled.
Guidelines for Sample Collection and Processing
Proper sample collection and handling are crucial for accurate thyroid test results. Follow these guidelines to ensure sample integrity:
- Collect blood into a plain red-top collection tube.
- Allow sufficient clotting time before centrifugation to maximize serum yield and prevent fibrin formation.
- After centrifugation, transfer the serum into a suitable vial for shipping or freezing. Freezing is recommended unless samples are shipped on the same day.
- Ship samples with cold packs to maintain a chilled temperature during transit. While freezing isn’t mandatory, samples should arrive chilled. Special Note: Avoid warming samples, especially during summer, as heat can falsely elevate Free T4 levels by causing T4 to detach from carrier proteins.
By understanding these diagnostic tests and adhering to proper sample handling procedures, veterinarians can effectively diagnose and manage feline hyperthyroidism, improving the health and well-being of their feline patients.
