A healthy thyroid produces the following hormones: T4, T3, T2, T1 and calcitonin.  T4, also called thyroxine, is the storage hormone, or the hormone stored away for future use. T4 is meant to convert to the active hormone T3.

Free T4 measures the amount of T4 that is not bound to proteins, and that which is useable.

That is in contrast to testing just the T4, also called the Total T4, which measures the total amount of T4 in your blood–both the T4 attached to proteins and thus unuseable, and that which is not attached, and thus useable.  Proteins helps transport T4 through your blood.

***To read what thyroid patients have learned in interpreting this test, go here.