Several different methods are used to estimate the free fraction of the total T4 or T3. Most commonly, the serum of patients is incubated with 125I-labelled T3 which occupies binding sites on the thyroid hormone binding proteins, mainly TBG, not already occupied by T4, while the remainder is left in solution. The protein-bound and free fractions are separated by addition of a resin, to which the free hormone binds. The proportion of free to protein-bound l25I-T3 is inversely related to the residual unoccupied binding capacity of thyroid hormone binding proteins, and is directly related to free hormone fractions.
