therefore, it must be converted to T3. Of the T3, either
produced by the thyroid or converted (mostly
by the liver) from FT4, some of that, too, will be bound by protein and unusable. So again, you need to make sure the Free T3 gets measured, because that's how much is actually available for the cells to use. It's usually only a small portion of the total T3. Free T3 levels correlates with symptoms, while TSH and FT4 do not.