Measurement of the Plasma Free Thyroxine Level as a Test of Thyroid Function

The concept that the small amount of unbound or free thyroxine (T4) is the factor in determining the true thyroid status of the individual (Robbins and Rall, 1957) has been supported by evidence in recent studies (Christensen, 1960; Sterling and Hegedus, 1962 ; Lee et al., 1964 ; Ingbar et al., 1965). Nevertheless, the measurement of either the plasma protein-bound iodine (P.B.I.) or the resin uptake of labelled triiodothyronine (T,) should suffice to confirm a diagnosis in most cases of primary myxoedema and primary thyrotoxicosis (Wellby and O'Halloran, 1966).