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Determination of Protein-Bound Iodine (PBI) In Human Plasma by Thermal Neutron Activation Analysis²

Baltimore

ABSTRACT

Neutron activation analysis was found to be a precise method for measuring PBI in plasma. Passage of human plasma through ion exchange resins in the ammonium acetate form prior to neutron activation removed greater than 99.9 per cent of the inorganic iodide as well as most other inorganic ions. Neutron bombardment of the treated plasma in a nuclear reactor with a flux density of approximately 10¹¹ thermal neutrons/cm² sec activated I¹²⁷ to the radioactive isotope I²⁸. After irradiation, major radiochemical impurities were eliminated by simple radiochemical technique (stable isotope dilution and differential precipitation). The induced I¹²⁸ radioactivity was measured and compared to activated iodide standard by gamma-ray spectrometry using spectrum stripping to eliminate interference from remaining radioactive contaminants. The activation analysis technique yielded results which were systematically 1.8 µg/100 ml higher than the Barker chemical method although thyroxine recovery studies run on both procedures were identical. This systematic difference could not be attributed to any systematic error in the activation analysis procedure indicating this technique may be measuring an additional iodine component of plasma such as iodotyrosines. Advantages over chemical methods of measuring PBI are elimination of the protein digesting step, elimination of the reagent blank and a reduction in the problem of iodine contamination.

FOOTNOTES

² This project was supported in part by USPHS Research Grant No. A-6155.

¹ The Johns Hopkins Medical Institutions.