A method for the direct estimation of total serum thyroxine
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Serum is deprotenized using 95% ethanol. The T4 thus released from the protein binding is quantitated by its competition with a fixed amount of T4123I for binding sites on a fixed amount of TBG. The mean value for thyroxine iodine in a group of 72 clinically euthyroid subjects is 5.94±1.34 g/100mls. giving a normal range of 3.3–8.6 μg/100mls. Agreement with the elinical status in a group of 130 subjects under assessment is 96.2%.
As well as providing a direct measurement of circulating thyroxine the method is not affected by the contaminants which invalidate the P.B.I. estimation.