A T4 or thyroxine test is a blood test that helps the physician diagnose thyroid conditions. The thyroid is a small gland at the base of the throat that produces hormones that control how the body uses energy.
These hormones affect metabolism, muscle strength, body temperature, and the nervous system.
Free T4: is the active form of thyroxine hormone that enters the tissues where needed.
Bound T4: is thyroxine attached or bound to certain proteins which prevent it from entering the tissues.
T4 levels can be measured with either a free T4 test or a total T4 test.
The free T4 test directly measures the amount of free T4 in the blood. It is considered that this test provides more accurate information than a total T4 test, so it's used more often.
The total T4 test measures free and bound T4 together. If using total T4, a calculation should be performed to figure out how much T4 is free.
Too much or too little T4 can signify thyroid disease.
Other situations when the test can be ordered are disorders of the pituitary gland, a gland at the base of the brain that makes the TSH hormone (thyroid stimulating hormone).
TSH controls how much T4 the thyroid gland produces. If the results from a TSH test are not normal, a T4 test can help diagnose whether the problem is with the thyroid or the pituitary gland.
This is a single-marker test measuring free T4. It is used to evaluate thyroid function.