Thyroid test

Out of the total T3 and T4 in circulation, most of it remains bound to thyroid binding globulin *, thyroid binding prealbumina nd Thyroid binding albumin. (*note – this is not thyroglobulin)
Only about 0.05% of each T3 and T4 remains free in circulation. This is FT3 and FT4.
These are better indicators for thyroid function than total T3 and Total T4. (total=bound+free).
For example in pregnancy, level of thyroid binding globulin rises; hence though total T3 and total T4 remains same, level of FT3 and FT4 decreases. Notes on Renal function tests.
Concept of FT3 and FT4