Summary Aim: We investigated whether additional application of “cold” iodine after therapy with radioiodine could result in a prolongation of the effective half life of iodine-131 and would thus lead to an increase of the effective thyroid radiation dose. Methods: Time-activitycurves after therapy with radioiodine were analysed in 25 patients (16 women, 9 men). Nine patients suffered from autonomously functioning thyroid nodules, 5 from autonomous multinodular goiter and 11 from Graves’ disease. These patients had an effective half life shorter than 4 days resulting in an undertreatment of > 20% with respect to the desired effective thyroid radiation dose. 2-4 days after therapy with radioiodine all patients received “cold” iodine for three days in a dose of 3 × 200 μg per day. Results: In 14 of the 25 patients an increase of the effective half life was observed. Patients with an autonomously functioning thyroid nodule showed a mean increase of the effective thyroid radiation dose of 40 ± 44 Gy, patients with toxic multinodular goiter of 29 ± 30 Gy and patients with Graves’ disease of 37 ± 37 Gy. Conclusion: Additional application of “cold” iodine after therapy with radioiodine can prolong the effective half life in selected patients. We suspect a correlation with the thyroid iodine pool. This will be the basis for further investigations hopefully resulting in a better patient preselection to determine who might respond to this therapy.

SummaryAim: Premedication with antithyroid drugs (ATD) compared to patients not pretreated with ATD causes a higher failure rate of radioiodine therapy (RITh) or demands higher therapeutical dosage of radioiodine (Rl). For clinical reasons and because of accelerated iodine metabolism in hyperthyreosis a compensated thyroid metabolism is desirable. Aim of this study was to investigate the influence of ATD on the biokinetics of Rl in case of Graves’ disease in order to improve RITh of patients pretreated with ATD. Methods: 385 consecutive patients who underwent RITh because of Graves’ disease for the first time were included: Group A (n = 74): RITh under continuous medication with ATD; Group B (ç = 111): Application of Rl under continuous medication with ATD, in case of insufficient Rl-uptake or shortened effective Rl-half-life ATD were stopped 1-5 days after RITh; Group C (n = 200): ATD were stopped 2 days prior to RITh in all patients. We examined the influence of ATD on Rl-uptake and effective Rl-half-life as well as the absorbed dose achieved on the thyroid in dependence of thyroid volume and applied Rl-dosage [TEQ - therapy efficiency quotient, (2)]. Results: In the Rl-pretest (all patients under ATD) the Rl-uptake was comparable in all three groups. During RITh Rl-uptake, effective Rl-half-life and therefore the TEQ were significantly higher in Group C as compared to Groups A and B (ñ <0,001, respectively). In Group B the medication with ATD was stopped in 61 of 111 cases 1-5 days after RITh. In this subgroup the effective Rl-half-life increased from 4,4 ± 1,7 d to 5,1 ± 1,6 d after stopping of ATD (ñ = 0,001). Conclusion: Stopping of ATD 2 days prior to RITh leads to an increased efficiency of about 50% compared to RITh carried out under ATD and therefore to a clear reduction of radiation exposure to the rest of the body with equal absorbed doses of the thyroid. Stopping of ATD shortly after RITh increases efficiency in case of short effective Rl-half-life, but it is inferior to stopping ATD 2 days prior to RITh.