AGREAT many studies have been carried out on the kinetics of iodine in man and a number of models have been proposed (1–9). The most common model has been a three-compartmental one in which iodine in the plasma is taken up by the thyroid and released back into the circulation as thyroid hormone. This in turn gets degraded and the released iodide recycles again. Excretion pathways from plasma iodide into urine and from plasma hormone into feces are also included. During the past ten years we have carried out over 100 kinetic studies on about 50 individuals in various states of thyroid function and found that the three-compartment model is inadequate to explain the data. In a previous publication (7), some of the computational problems encountered in connection with the data analysis were discussed and preliminary attempts to evolve a consistent model were presented. Further modifications of the model were introduced more recently. This paper is intended to summarize the present state of the model, the argu...