This article is from the Interpretation Of Lab Test Profiles, by Ed Uthman firstname.lastname@example.org with numerous contributions by others.
This is a convenient parameter with mathematically accounts for
the reciprocal effects of T4 and T3 uptake to give a single
figure which correlates with free T4. Therefore, increased FTI
is seen in hyperthyroidism, and with decreased FTI is seen in
hypothyroidism. Early cases of hyperthyroidism may be expressed
only by decreased thyroid stimulation hormone (TSH) with normal
FTI. Early cases of hypothyroidism may be expressed only by
increased TSH with normal FTI.