T3 uptake (T3u) -Triiodothyronine (T3) and thyroxine (T4) are two of the most common hormones released by the thyroid. They exist in two forms: Either attached to a carrier protein or not. This carrier protein is known as thyroxine-binding globulin (TBG), which like the name implies, is a protein that transports the hormone throughout the body. Only the T3 and T4 hormones that are not bound to TBG, or “free” hormones, affect the body’s processes.
The purpose of taking the T3 resin uptake test is to help estimate the amount of TBG in the blood, and how much of the T3 and T4 in the body is free from the carrier proteins. A high T3u means thyroxine binding proteins are more saturated than normal. Measuring T3u does little good alone; it needs to be combined with the results of a total T4 test to get Free Thyroid Index (T7).
Total T4 -The T4 Total Thyroxine test determines the total amount of thyroxine (T4) in the blood. Both the thyroxine bounded to thyroxine-binding globulin (TBG) and the free thyroxine are accounted for in this test.
T7 (Free T4 or Free Thyroid Index) - When T4 is bound to protein, it prohibits it from entering the various tissues that need the hormone. Free T4 is the version of T4 that can get to the tissues it needs to get to. This test measures the amount of free T4 in the blood. It’s measured by combining the T3u and the T4. The free T4 is the most accurate measure of how well the thyroid is working.