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

Abstract

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 125 I 2 or 125 I − , blood 125 I levels were maximal at 2 hr and reached similar concentrations in fed animals receiving 125 I − and fasted animals receiving either 125 I 2 or 125 I − . However, when 125 I 2 was administered to fed animals the initial levels of 125 I into blood were significantly lower than after the other treatments. The half-life of elimination of 125 I from the blood appeared independent of the form of iodine administered. The initial distribution of 125 I 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 I 2, this may largely be explained by the increased retention of 125 I in the stomach contents. In fasted animals, both stomach content and blood levels of 125 I were similar whether I 2 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 I 2. 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 125 I − than pretreatment with equivalent concentrations of I 2.