Abstract
Thyroxine (T<sub>4</sub>) in serum was determined by competitive protein-binding analysis, the resin uptake of liothyronine I 125 (RT<sub>3</sub>) was measured, and a "free T<sub>4</sub>index" was calculated as the mathematical product of values for T<sub>4</sub>iodine (T<sub>4</sub>I) and RT<sub>3</sub>. Pregnant euthyroid patients and normal women taking estrogens had high values for T<sub>4</sub>I associated with low values for RT<sub>3</sub>. Sick euthyroid patients tended to have decreased values for T<sub>4</sub>I with high values for RT<sub>3</sub>. The free T<sub>4</sub>index accurately reflected the state of thyroid function in euthyroid patients with disturbances of T<sub>4</sub>-binding proteins, as well as in patients with hyperthyroidism or hypothyroidism.
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