Colonic excretion of iodide in normal human subjects

Patterns of fecal radioiodine excretion were studied in seven normal young men with intact, unblocked thyroid glands, who received repeated oral daily doses of [125I]iodide in an attempt to achieve isotopic equilibrium. Fecal radioactivity was assumed to have originated either in the circulating inorganic radioiodine (radioiodide) compartment or in the circulating hormonal protein-bound iodine (PBI) compartment. Activity in the PBI compartment was measured directly in serum samples from which radioiodide had been removed by dialysis or resin treatment. Activity in the radioiodide compartment was measured by the difference between total and hormonal radioiodine, and also as a projection from the rate of urinary excretion of radioiodine. These compartments were fitted to the observed sequential fecal radioiodine data in each subject to identify the origins of the fecal radioactivity, using the SAAM modeling program. The fraction of fecal radioactivity attributable to iodide was 0.55 +/- 0.35 (mean +/- SD) (geometric mean 0.44, range 0.25-0.96). In all cases, at least some contribution from the iodide compartment was required for model fit to the observed pattern of fecal radioiodine excretion. These data demonstrate that, despite long-existing opinion to the contrary, iodide is an important component of intestinal iodine excretion in humans. This finding explains the presence of colonic activity in postradioiodide images of athyreotic patients.