Determination of Protein-bound Iodine*

The growing interest in chemical evaluation of thyroid function by use of the plasma level of protein-bound iodine (PI) is attested by the increasing numbers of reports in which this criterion is employed, both chemically and experimentally. However, the specific techniques available for such a determination do not yet seem sufficiently reliable. The purpose of this communication is to describe a procedure evolved in this laboratory which is consistent and sensitive in our hands. The complete determination of plasma PI involves four distinct steps: (1) precipitation and washing of the plasma proteins; (2) digestion of the protein, leaving the PI in inorganic form; (3) distillation of the inorganic iodide; (4) actual determination of the inorganic iodide. The shortened technique of Salter (4), in which a dry ashing with NaOH and Na&Oa was used to eliminate the need for distillation, has not proved satisfactory in our experience, and no other short cuts have been proposed. In 1940, Chaney (1) described an all-glass still for use with a chromicsulfuric acid digestion mixture; iodide catalysis of the reduction of ceric ions by arsenious acid was employed for the ultimate calorimetric determination of iodine. Although other workers have adopted the Chaney still (7), the highly sensitive ceric-arsenious catalysis method was generally avoided until Taurog and Chaikoff reported an entire procedure for plasma iodine (8). The iodide catalysis reaction had been extensively studied in 1937 by Sandell and Kolthoff (5), who demonstrated a marked enhancement of the iodide effect by the presence of a relatively high concentration of chloride. We have found that the use of chloride in optimum concentration improves the sensitivity 30 to 80 per cent with no sacrifice in reproducibility. None of the recently reported techniques has taken advantage of this appreciable increase in sensitivity.