Protein-Bound Iodine Determination with Stable End Point

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Abstract
Observations over a 4-year period on the serum protein-bound iodine determination, using the chloric acid digestion technic in a nonautomated laboratory, have been reported. Various steps in the procedure have been investigated, indicating the feasibility of eliminating the washing of protein precipitates. It was found that during precipitation with trichloroacetic acid an average of 5.4 per cent of added radiothyroxine was lost, whereas an additional loss of 10.3 per cent of labeled hormone occurred because of washing of the precipitate. These findings, coupled with other presented, have indicated that washing of precipitates could be eliminated. Laboratory error can be reduced by the use of cooling between the various steps in the colorimetric reaction and stabilization of the end point with brucine. The various modifications have simplified the precedure so that a run can be performed easily over a 7-hr. period. In this study 609 euthyroid individuals had a mean PBI of 5.48 [plus or minus] 1.08, s. d.; in 32 hypothyroid patients and in 28 hyperthyroid patients, it was 1.41 [plus or minus] 0.81 and 12.19 [plus or minus] 2.98, respectively.