Therefore when mixed with T3 (which is easier to measure), the blood proteins would take up very little T3. Thus a low T3
uptake implies elevated T4 levels and vice versa. Thus the T3
Uptake test is actually an antiquated, inaccurate way to measure T4 levels.
* Free Thyroxine Index: This test uses T3 Uptake as one of its factors and therefore is as useless as the T3 Uptake. Again, I list it here only as a warning to not throw your money away on this and save it for a meaningful test.