Bound Iodine and Binding Protein

Abnormal values of protein-bound iodine (PBI) are often due not to thyroidal disorders but to incidental changes in serum iodine or serum proteins. The artifacts, caused by ingested iodides or by iodine-containing preparations used in radiography, are generally well known. Less well recognized is the interference with PBI values by changes in the composition or in the binding capacity of thyroxinebinding protein. Unlike serum changes induced by iodine which are always traceable to extrinsic causes, changes in thyroxine-binding proteins may be due to either extrinsic or intrinsic factors. To the former category belong the increases in serum protein saturation capacity for thyroxine which occur during pregnancy, after estrogen administration, and in some women with breast cancer. Intrinsic causes are attributable to genetically determined changes in the thyroxine-binding protein.

Beierwaltes and Robbins¹ first reported the occurrence of an elevated PBI and total thyroxinebinding capacity in an otherwise normal man and