Effect of cimetidine and ranitidine on absorption of [125I]levothyroxine administered orally

Female patients with a simple goiter were pretreated on 2 occasions (at an interval of 4 wk) with po placebo or 400 mg cimetidine (Cim) (Group A, n = 10), or with placebo or 30 mg ranitidine (Ran) (Group B, n = 10), 90 and 150 min, respectively, prior to the po gelatin capsules containing [125I]levothyroxine ([125I]LT4). A double-blind randomized study protocol was kept. Venous blood samples were taken at 15, 30, 45, 60, 75, 90, 105, 120, 150, 180, 210, and 240 min after po [125I]LT4 and the radioactivities in serum were counted. Similar [125I]LT4 radioactivities were found after placebo pretreatment in both groups: AUC 467 +/- 82 in Group A vs 459 +/- 109 in Group B. Cimetidine decreased the serum [125I]LT4 radioactivities: AUC371 +/- 72 (Cim) vs 467 +/- 82 (placebo) (P < 0.01), but Ran did not: AUC 477 +/- 132 (Ran) vs 459 +/- 109 (placebo) (P > 0.05).