Thyroxine (T₄) in serum was determined by competitive protein-binding analysis, the resin uptake of liothyronine I 125 (RT₃) was measured, and a "free T₄index" was calculated as the mathematical product of values for T₄iodine (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 T₄index 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.
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Raj K. Sarin (1970) studied this question.