The free thyroxine (T4) direct serum test from Walk-In Lab helps to evaluate thyroid gland function.The free thyroxine test is a blood test and does not require fasting. Results are typically available within one to two days. The test is also known as a free T4 test, free thyroxine test, and unbound T4 test.The free T4 test is a newer counterpart to the total T4 test and has generally replaced its use. Both are used to help diagnose hyperthyroidism and hypothyroidism. The total T4 test results are affected by the amount of proteins available in the blood to bind the hormone whereas the free T4 test results are not affected by the amount of proteins in the blood. Since free T4 is the active form of thyroxine, the free T4 test is widely considered to be the more accurate assessment. A free T4 test is generally ordered following an abnormal result from a TSH test or ordered along with a TSH test in order to get an accurate assessment of thyroid function.The free T4 test is generally ordered to help evaluate a person with a goiter (enlarged thyroid gland) and to aid in the diagnosis of female infertility. The test results of a free T4 test are not diagnostic but help to guide a doctor in performing additional testing to find the root cause of the shortage orexcessinthyroidfunctionality.