Thyroxine (T4)
Thyroxine (T4) :
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. 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 patient’s serum). 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
