Protein-bound iodine determination by activation analysis.

The most commonly-used test for determining the serum concentration of thyroid hormones is the protein-bound iodine analysis (PBI) which measures the serum's entire organic and some inorganic iodide concentration. Since it is desired to measure only that portion of the serum iodine which is in the thyroxine molecule, this test may be misleading. The method of PBI determination rou tinely used in most clinical laboratories is the dry ash method of Barker (1). It is one of the more complicated of the commonly-used clinical laboratory tests requiring skilled technicians and generally requiring a separate laboratory be cause of contamination problems. The presence in serum of organic iodine compounds used for radiographic and therapeutic purposes precludes the deter mination of PBI by the dry ash method for periods up to years (1). The butanol-extractable iodine (BEI) method of serum iodine determina tion is recognized to be more reliable than the PBI, but it is also more difficult to perform and is therefore used less frequently (2,3). Pileggi (3) has described a method of serum iodine determination using an ion exchange resin column which is reliable for the determination of thyroxine iodine in the presence of elevated inorganic iodine or iodotyrosine levels. When this method was compared with the PBI method, the column method also showed superiority in its ability to distinguish thyroxine iodine from organic iodine compounds. This method, subsequent to the ion column separation, suf fers from the usual PBI analytic problems.