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Determination of Serum Thyroxine by Saturation Analysis of Thyroxine Binding Proteins

Hospital General del Centro Médico Nacional, I.M.S.S., Mexico City, Mexico

Correspondence: For reprints contact: Alfredo Cuarón, Tepeji 21, Mexico 7, D.F., Mexico.

ABSTRACT

A new method for estimating thyroxine concentration in serum based on the saturation of the thyroxine-binding proteins is presented. This procedure was assessed in sera from 487 patients and proved to be simple, accurate, economical and fast. It is not affected by iodine contamination since it is not based on chemical estimations of this element. It offers a direct measurement of thyroxine and is specific for this hormone. Its methodology is suitable for a small nuclear medicine unit equipped with a conventional well scintillation counter since it does not involve any chemical procedure and is simple enough to be performed by any person used to basic radioisotope methodology. It is a low-priced method compared with those previously reported since a Sephadex column is cheaper than the resin sponge and the same column can be used several times during a long period of time. Furthermore, this procedure uses the same labeled hormone used by the traditional in vitro thyroid tests and consequently does not increase costs by requiring the purchase of a wider variety of radiopharmaceuticals. It seems to be more accurate than the methods based on the use of a resin sponge because this procedure automatically excludes the radioactive iodide contaminating the stock solution of labeled triiodothyronine. Since it is an in vitro test, it does not expose the patient to ionizing radiation. Finally, its overall diagnostic accuracy is higher than that of the protein-bound iodine estimation when used during routine clinical work due to the high incidence of invalidation of protein-bound iodine estimation caused by iodine ingestion.