In a previous report (1) the disappearance curves of 181-11Abeled L-thyroxine and L-triiodothyronine were described in euthyroid subjects. The radioactive compounds were injected intravenously, and the radioactivity of the plasma or se- terminations. were carried out2 (4, 5). The BEI was done to avoid interference from inorganic iodine previously administered in the three instances where this had occurred. The two determinations were used interchangeably in the calculations of "organic iodine" figures; PBI ordinarily exceeds BEI by approximately 0.6,ug. per cent in the absence of inorganic iodine (5). Calculations T-he injection of radiothyroxine was followed by a relatively rapid initial fall in radioactivity of the plasma (Figure 1). This was attributed to diffusion of the tracer out of the vascular compartment as it was distributed throughout the body's extrathyroidal organic iodine (EOI) pool. After two days the plasma radioactivity declined more gradually, resulting in a straight line when plotted seniilogarithmically. This slow exponential com-

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