Total T4 (TT4)

There are two forms of thyroid hormones: the bound T4 (bound to Total Binding Globulin (TBG) and the free T4 (active form of thyroid hormone, present in small amounts). TT4 measures both the bound and free forms.

T3 Uptake

This measures the unbound thyroxine binding globulin (unsaturated with thyroid hormone). When TBG is increased, T3 uptake is decreased, and vice versa.

Free Thyroxine Index (FTI)

The FTI is obtained by multiplying the (Total T4) times (T3 Uptake) to obtain an index. The FTI is considered a more reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding.


References:
http://www.webmd.com/a-to-z-guides/thyroid-hormone-tests
http://www.dundee.ac.uk/medther/tayendoweb/images/thyroid_function_tests.htm
http://www.labtestsonline.org/understanding/analytes/t3/test.html