Influence of the Gastrointestinal Iodide Cycle on the Early Distribution of Radioactive Iodide in Man *

Abstract

The salivary-gastric-enteric cycle of radioactive iodide was studied during the first 3 hrs. after isotope administration in 9 normal subjects. Radio-iodide in the thyroid, plasma, urine, and pooled salivary-gastric secretions was measured frequently. Control sessions for each subject were identical, except that no secretions were aspirated from the gastric tube. Radioiodide recovered from the stomach averaged 23% of that administered (range, 11 to 44%). Thyroidal clearance averaged 16 ml/min.; renal clearance, 34 ml/min.; and gastrosalivary clearance (not corrected for dead space), 46 ml/min. When gastric contents were removed, the nonthyroidal, nonurinary volume of distribution remained constant (averaging 20.1 1 or 27% of body weight) after the 1st hr., and plasma radioactivity declined along a simple exponential curve. The rate of this decline could be accounted for by the summed thyroidal, renal, and gastrosalivary clearance rates. In the control sessions, the increase in volume of distribution persisted longer, and in 4 subjects it was still evident at 180 min. Thigh/plasma ration became constant after 1 hr. but was not so good a predictor of nongastrointestinal volume of distribution as was body weight. Curves of plasma radioactivity in the control sessions varied greatly among the individuals and did not follow a simple exponential function. This difference must be due to the effect of intestinal reabsorption of radioiodide previously cleared.