Radioiodine therapy and thyrostatic drugs and iodine

Radioiodine therapy is now the most common definite treatment for persistent hyperthyroidism. The outcome of radioiodine therapy depends mainly on the absorbed energy dose in the diseased thyroid tissue. The administered activity and the resulting target dose in the thyroid depend on both the biokinetics of radioiodine and the actual therapeutic effect of radioiodine in the thyroid. Thyrostatic drugs have a major influence on the kinetics of radioiodine in the thyroid and may additionally have a radioprotective effect. Pre-treatment with thyrostatic medication lowers the effective half-life and uptake of radioiodine. This can reduce the target dose in the thyroid and have a negative influence on the outcome of the therapy. Discontinuation of medication shortly before radioiodine administration can increase the absorbed energy dose in the thyroid without increasing the whole-body exposure to radiation as much as would a higher or second radioiodine administration. Furthermore, administration of non-radioactive iodine-127 2-3 days after radioiodine administration can also increase the effective half-life of radioiodine in the thyroid. Thus, improving the biokinetics of radioiodine will allow lower activities to be administered with lower effective doses to the rest of the body, while achieving an equally effective target dose in the thyroid. (orig.)