Thyroxine (T4) in serum was determined by competitive protein-binding analysis, the resin uptake of liothyronine I 125 (RT3) was measured, and a "free T4 index" was calculated as the mathematical product of values for T4 iodine (T4I) and RT3. Pregnant euthyroid patients and normal women taking estrogens had high values for T4I associated with low values for RT3. Sick euthyroid patients tended to have decreased values for T4I with high values for RT3. The free T4 index accurately reflected the state of thyroid function in euthyroid patients with disturbances of T4-binding proteins, as well as in patients with hyperthyroidism or hypothyroidism.
Sarin RK, Anderson BG. Serum Thyroxine Resin Uptake of Liothyronine I 125, and Free Thyroxine Index. Arch Intern Med. 1970;126(4):631–634. doi:10.1001/archinte.1970.00310100077008