Free thyroxine. An abbreviated practical assay.

Abstract

Free or unbound thyroxine (FT4) in serum comprises less than .1% of total circulating thyroxine. This small fraction is thought to be responsible for the physiologic action of thyroxine. Protein-bound iodine (PBI) butanol-extractable iodine (BE1) T4 by column and T4 by competitive protein binding are all measurements of total T4 in serum. They usually reflect thyroid status. However during pregnancy and with estrogen therapy total T4 is elevated and is not an accurate measure of thyroid disease as FT4 levels remain normal. This report describes a simplified rapid reproducible procedure for determining FT4 in serum. A 3.5-hour dialysis technic in which serum was diluted and labeled with T4-125 iodine was used. Subsequent purification of the dialysate was done with protein-coated charcoal. The shortened dialysis period eliminated the effects of bacterial contamination noted by longer methods. The protein-coated charcoal purification of the dialysate is less complicated than those previously described. Results compared favorably. Details of preparation of reagents and technics employed are given. Blood specimens were obtained from hyperthyroid and hypothyroid patients pregnant women women using oral contraceptives and euthyroid women sick with diseases not known to affect thryoid function. Diluted radioactive serum was prepared for each patient. Repeated freezing and thawing of serum samples with repetition of tests had no significant effect on results. Values between the various groups of patients agree with those found by other workers. Thyroid function in pregnant women and in women using oral contraceptives is shown by FT4 not total T4. The method described is suitable for routine use in clinical laboratories.