Total T4(Thyroxine) Test

A T4 blood test measures the amount of the T4 hormone in the blood. T4 is produced by the thyroid gland. T4 levels are important, because T4 increases numerous enzymes that produce energy for the body. Alternative Name: Thyroxine test.

Thyroxine (T4) Test

What is a Thyroxine (T4) Test?

A thyroxine test is a blood test that helps diagnose thyroid conditions. The thyroid is a small, butterfly-shaped gland at the base of your throat. Your thyroid makes hormones that control the way your body uses energy. These hormones affect your weight, heart, body temperature, muscle strength, and even your mood. In children, thyroid hormones affect growth, too.

Thyroxine, also known as T4, is a type of thyroid hormone. A T4 test measures the level of T4 in your blood. Too much or too little T4 can be a sign of thyroid disease.

There are two forms of T4 in your blood:

• Free T4 is the active form of thyroxine hormone that enters your tissues where it's needed.
• Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your tissues.

T4 levels can be measured with either a free T4 test or a total T4 test:

• A free T4 test directly measures the amount of free T4 in your blood. Medical experts believe this test provides more accurate information than a total T4 test, so it's used more often.
• A total T4 test measures free and bound T4 together. A calculation may be done to figure out how much T4 is free. This is a less accurate way to check how well your thyroid is working.

Other names: free thyroxine, free T4, total T4 concentration, T4 index, thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis