CLINICAL DETERMINATION OF PROTEINBOUND IODINE IN PLASMA*†

Abstract

IN THE evaluation of thyroid disease, knowledge of the level of circulating thyroid hormone, as determined by protein-bound iodine (PBI), is an invaluable objective measure. During the past four years, use of a procedure involving chromic acid digestion coupled with distillation for doing the PBI work has proved too involved and time-consuming for many laboratories. Attention has, therefore, been redirected towards making the simpler alkali-incineration technique sufficiently consistent and dependable to enable its application to clinical material. The method evolved is more restricted in its applicability than the acid digestion-distillation technique but is equally reliable and far more convenient to carry out on a routine basis. Table 1 compares the two general procedures, and it can be seen that, although steps 1 and 4 are essentially identical, great savings in time and manipulation are achieved in steps 2 and 3. With an automatically controlled furnace, the dry-ashing does not require close attention, and the difficult distillation (step 3) has been eliminated. This latter simplification.entails the danger that nonvolatile substances eluted from the ash may take part in the final sensitive colorimetric determination of iodide catalysis of the arsenious acid-ceric sulfate reaction. Interference with the reaction can be ruled out by the addition of standard iodide to an aliquot of each sample.