Serum hormonal iodine tests are, at best, only indirect indices of thyroid function. The level of protein-bound iodine (PBI) may be influenced by extrathyroidal factors, which affect the metabolism of either its iodine or its protein moiety. The vulnerability of the PBI test to vitiation by iodine-containing drugs or iodinated radiopaque substances used in roentgenographic diagnosis is well known, as is the elevation of PBI during pregnancy, when thyroxine-binding globulin capacity is increased, and thyroxine turnover diminished.
Less familiar are the poorly understood vagaries of protein metabolism which affect iodine binding. Their study is made difficult by the strong probability that more than a single specific protein may be implicated. Imarisio and Greco,1 using a flow dialysis technique, demonstrated that not only thyroxine-binding globulin but all serum proteins bind thyroid hormone to a greater or lesser degree.
Rall and Conard2 report an interesting disorder of iodine-carrying protein among
PROTEIN-BOUND IODINE. JAMA. 1966;197(11):918–919. doi:10.1001/jama.1966.03110110142037