Many cats become hyperthyroid as they age. Thyroid levels are also affected by nutrition, disease states (particularly liver and kidney disease), medications, etc. The most comprehensive panel offered by the laboratory is the Feline Thyroid Panel, which includes T4, T3, free T4 by equilibrium dialysis and TSH. This would be the most useful in assessing a cat with many health problems.

T4 & T3

The T4 (total T4 or thyroxine) test is a diagnostic test for hyperthyroidism. It is also useful as a screening test for hypothyroidism and for monitoring treatment with methimazole. T4 concentrations can be affected by medications, disease states, and nutrition. T3 (total T3 or triiodothyronine) is the active form of thyroid hormone. T3 levels are usually elevated or high-normal in hyperthyroid cats. 

Free T4 by equilibrium dialysis

Free T4 (FT4) is the non-protein bound thyroxine present in the blood. Measuring FT4 can help determine if cats with high-normal or borderline total T4 levels are normal or hyperthyroid. The equilibrium dialysis (ED) method is the only assay used for testing feline samples in this lab.  FT4 ED levels can be falsely elevated by mishandling specimens/samples (e.g. warming, severe hemolysis, etc.).  FT4 concentrations may also be affected by low protein states caused by kidney or liver disease, neoplasia, etc.  We have determined the Free T4 Two Step (TS) method used for dog samples is not valid for cats. 

TSH Baseline

TSH (Thyroid Stimulating Hormone) measurement can be used in combination with the other thyroid tests mentioned. Hyperthyroid cats will generally have low levels of TSH. Hyperthyroid cats showing clinical signs of hypothyroidism after treatment with radioactive iodine will generally have high TSH levels.

T3-Suppression Test for Cats

Diagnosis of feline hyperthyroidism can usually be accomplished by measuring T3, T4 , and/or free T4 in a single baseline blood sample. The T3-suppression test is used for diagnosing hyperthyroidism in cats that have normal T3 , T4 , and/or free T4 concentrations but appear clinically hyperthyroid.

1. Draw a baseline blood sample following the collection and processing procedure below  and  freeze  this  sample.
2. Orally administer 25 μg of T3 (e.g., Cytobin®) every 8 hours for 2 days; administer the seventh dose on the morning of the third day.
3. Draw another blood sample 4 hours after the last dose of Cytobin®. Process as before.
4. Submit the paired serum samples together, optimally shipped so they are received chilled.

Guidelines for Sample Collection and Processing

1. Collect blood into a plain red-top collection tube.
2. Allow blood adequate time to clot prior to centrifugation to ensure sufficient yield and avoid fibrin formation.
3. After centrifugation, transfer the serum into a vial suitable for shipping or frozen storage. Frozen sample storage is recommended unless samples are being shipped the day taken.
4. Ship samples with cold packs. A frozen specimen is not necessary, but the sample should arrive chilled.
   Note: Free T4 can be falsely elevated if the sample is warmed. Heat causes dissociation of T4 from its carrier proteins increasing free T4 concentrations. Special care should be taken when shipping samples in the summer.