Iodine contamination of the serum protein-bound iodine: incidence and clinical significance.

Iodine contamination as defined by the combination of a raised (PBI-T(4)I) difference and low (131)I neck uptake was found in 38 (17.5%) of 217 euthyroid patients. Of these, 17 had PBI levels of greater than 20 mug/dl but in the remainder levels were clinically feasible. In only 21 was there a history of exposure to iodine. Two of 12 hypothyroid patients had PBI levels well within normal limits. False elevation of the PBI is thus shown to be common. It is neither always obvious nor can it be easily avoided. The PBI is not an acceptable alternative to T(4) estimation by other methods.