Measuring of Free and Unbound thyroxine levels in the blood.
Free T4 represents the amount of hormone that’s available for use by the cells of the body. Total T4 represents all the hormone present, but the Free amounts represent what is useable. Low levels of Free T4 may indicate hypothyroidism (underactive) and high levels may indicate hyperthyroidism (overactive).
Recommended with Free T4
Is Fasting Required? No
More Information: http://www.questdiagnostics.com/testcenter/TestDetail.action?ntc=866