Differences in the distribution of iodine and iodide in the Sprague-Dawley rat

Use of iodine as a drinking water disinfectant for extended space flight raises concerns about potential chronic effects on health. A key question is whether the chemical form of iodine might play a role. To address this question the influence chemical form has on the uptake and distribution of radioiodine was studied in fed and fasted rats. Following oral administration of 125I2 or 125I-, blood 125I levels were maximal at 2 hr and reached similar concentrations in fed animals receiving 125I- and fasted animals receiving either 125I2 or 125I-. However, when 125I2 was administered to fed animals the initial levels of 125I into blood were significantly lower than after the other treatments. The half-life of elimination of 125I from the blood appeared independent of the form of iodine administered. The initial distribution of 125I to the thyroid depended sharply on chemical form, being greater when iodide rather than iodine was administered, whether animals were fed or fasted. In fed animals administered I2, this may largely be explained by the increased retention of 125I in the stomach contents. In fasted animals, both stomach content and blood levels of 125I were similar whether I2 or I- was administered. Since thyroid uptake of iodine is specific for I-, this suggests that the form of iodine in the blood was different in animals administered I2. This notion was further supported by the finding that pretreatment of animals with varying concentrations of I- in drinking water was four times as effective in suppressing the uptake of a test dose of 125I- than pretreatment with equivalent concentrations of I2.