A healthy thyroid produces the following hormones: T4, T3, T2, T1 and calcitonin. T3, also called triiodothyronine , is the active hormone. T3 comes from the conversion of T4 to T3, and it’s also made directly by the thyroid.

Free T3 measures the amount of T3 that is not bound to proteins, and that which is useable.

That is in contrast to testing just the T3, also called the Total T3, which measures the total amount of T3 in your blood–both the T3 attached to proteins and thus unuseable, and that which is not attached, and thus useable. Proteins helps transport T3 through your blood.

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

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