Diagnosing Cat Thyroid Problems with the T4 Test

The T4 test is a blood test that measures the amount of thyroxine in the blood. Thyroxine, also known as T4 hormone, is produced by the thyroid gland along with another hormone known as triiodothyronine. Thyroxin is vital in the regulation of the cat’s growth and metabolism. An imbalance in the production of this hormone can lead to thyroid disorders.

The overproduction of this hormone leads to hyperthyroidism which increases the rate of metabolism in the cat leads to increased heart rate, weight loss, intolerance to heat, increased rate of breathing, and nervousness. It’s usually diagnosed in middle-aged to older cats. Underproduction of the thyroid hormone leads to hypothyroidism which in causes a decreased metabolic rate, decreased heart rate, weight gain, intolerance to cold, infertility, constipation, high blood cholesterol, dry skin and hair loss.

Particulars of Thyroid Tests:

  • Cats don’t require sedation
  • Blood samples are allowed to clot
  • Pets susceptible to excessive bleeding require extra care
  • The test may be conducted at an external laboratory

T4 Test

There are two T4 tests, namely, the total T4 test and the free T4 test. These help vets measure the amount of thyroxine in the cat’s body. These tests are useful for the diagnosis of both hyperthyroidism and hypothyroidism.

The Total T4 Test

This test is also known as the Baseline T4 Test and is the most commonly used test. The blood sample is subjected to a radioimmunoassay to measure the amount of T4 hormone in the blood. Based on this measured amount, the vet can decide whether the cat is suffering from hyperthyroidism, hypothyroidism or neither. The test measures the T4 hormone that’s stuck to blood proteins in the bloodstream as well as free floating T4 hormone in the blood. Thus, the results of the test can be affected by the protein levels in the blood. In addition, sick as well as older cats might have lesser amounts of the hormone in their bloodstream even though they may be suffering from hyperthyroidism. Hence, this test is not very accurate.

The Free T4 Test

This test is done by equilibrium dialysis. It’s not affected by protein levels in the blood as it only measures the free floating hormone in the bloodstream. It’s this free floating hormone that’s active and regulates the metabolism of the body. Thus, the free T4 test is more sensitive and is used instead of the total T4 test.

However, this test might show an elevated amount of the free hormone in the blood if the cat is sick from diseases other than thyroid disorders as such cats have elevated amounts of free floating T4 hormone in their bloodstream. Cats suffering from intestinal problems also exhibit weight loss as well as elevated amounts of thyroxine in their bloodstream. Several cats might be misdiagnosed as a result. Thus, cats shouldn’t be diagnosed with hyperthyroidism based on only the free T4 test.

The vet will advise carrying out both the total T4 and the free T4 tests together with the TSH test. These tests, in addition to a physical exam, help to determine the reasons and the type of thyroid disorder in the cat. Thyroid tests are usually advised in cases of goiter, infertility, in older cats that lose weight, and in cats with an enlarged thyroid gland.