This article is from the Interpretation Of Lab Test Profiles, by Ed Uthman email@example.com with numerous contributions by others.
This is a measurement of the total thyroxine in the serum,
including both the physiologically active (free) form, and the
inactive form bound to thyroxine-binding globulin (TBG). It is
increased in hyperthyroidism and in euthyroid states
characterized by increased TBG (See "T3 uptake," above, and
"FTI," below). Occasionally, hyperthyroidism will not be
manifested by elevation of T4 (free or total), but only by
elevation of T3 (triiodothyronine). Therefore, if thyrotoxicosis
is clinically suspect, and T4 and FTI are normal, the test
"T3-RIA" is recommended (this is not the same test as "T3
uptake," which has nothing to do with the amount of T3 in the
T4 is decreased in hypothyroidism and in euthyroid states
characterized by decreased TBG. A separate test for "free T4" is
available, but it is not usually necessary for the diagnosis of
functional thyroid disorders.