Kinetics of acute and chronic iodine excess

Iodine plays an important role in thyroid physiology resulting from its imp ortance as a requisite substrate for the synthesis of thyroid hormones and from its action as a regulator of thyroid function. Following intestinal ab sorption, inorganic iodide is largely confined to the extracellular fluid. Serum concentrations of inorganic iodide well reflect the amount of iodine present in the extracellular compartment. Since serum inorganic iodide leve ls are important determinants of thyroid iodine uptake, serum iodide measur ement offers a valuable tool for the investigation of many basic and clinic al aspects of thyroid iodine metabolism. Here we summarize important aspect s of iodine metabolism and focus selectively on technical aspects of serum inorganic iodide measurement and on the kinetics of inorganic iodide in var ious states of iodine excess. Presently, paired-ion, reversed-phase HPLC wi th electrochemical detection is obviously the best method for measurement o f serum inorganic iodide being highly sensitive, easy to perform, and almos t completely insensitive to interfering substances. Using this method, we c ould demonstrate an acute increase of serum inorganic iodide during the adm inistration of large amounts of iodide as Lugol's solution given preoperati vely in patients with Graves' hyperthyroidism. In patients under treatment with the iodine containing drug amiodarone (n=37), serum inorganic iodide l evels were highly elevated (range 3.5-208.2 mu g/dl, median 36.6 mu g/dl). Serum concentrations of inorganic iodide were correlated neither to the dai ly amiodarone dose, nor to the serum levels of amiodarone.