Seyed Rasoul Zakavi
Nuclear Medicine Research Center, Mashhad University of Medical Sciences, Mashhad, Iran
Hyperthyroidism is a form of thyrotoxicosis due to inappropriately high synthesis and secretion of thyroid hormones by the thyroid. Prevalence of hyperthyroidism is about 1% of general population in different parts of the world and the most common causes include Grave's disease, multinodular goiter or toxic adenoma. Radio-iodine therapy (RIT) with I-131 is the preferred method of treatment in toxic nodular goiter (single or multinodular) all around the world. It is also considered as effective and acceptable method of treatment in patients with Grave's disease. Treatment of Grave's disease may vary in different parts of the world. While RIT is used more commonly as first treatment modality in USA, surgery or antithyroid drugs (ATD) are chosen as the first treatment in other countries. Anyhow recurrence is very common after ATD therapy and RIT is usually considered for treatment of these patients in the course of therapy. Radio.iodine therapy is contraindicated in pregnancy, lactation and when there is high suspicion of thyroid cancer. Presence of Grave's ophthalmopathy and inability to comply with radiation safety recommendations are other factors that are considered in choosing optimal treatment method. In patients with highly elevated T4 and T3 levels, older age and severe symptoms of hyperthyroidism, pretreated with beta-blockers and antithyroid drugs are usually performed before radio-iodine therapy. While Beta-blockers have no effect on success of RIT and may be continued safely, discontinuation of ATD is necessary before RIT. In a recent clinical trial, we showed that 24-48 h of methimazole (MMI) discontinuation before RIT is as effective as longer term discontinuation. Although RIT has been used for more than 50 years, there is no consensus on the optimal dose of radio-iodine for treatment of hyperthyroidism. It is recommended that sufficient radiation should be administered in a single dose. Generally two approaches have been used for this purpose. One approach uses a fixed dose of I-131 for treatment of all patients. The other approach tries to calculate the delivered dose to the thyroid. Accurate determination of thyroid radiation dose requires measurement of effective half life of I-131 which is impractical as a routine clinical test. However postulating an average effective half life of I-131, an acceptable estimate of radiation dose to thyroid is achievable by measurement of thyroid weight and RAIU. Each of fixed or calculated methods of RIT may be further categorized to low or high dose subgroups. In a recent meta-analysis on toxic nodular goiter, we showed that calculated dose may be preferable to fixed dose method and could treat patients with lesser amount of I-131. Radiation safety precautions also should be fully explained to the patient to prevent unnecessary radiation of family and friends as well as to prevent radioactive contamination. It is equally important to prevent irrational fear of radiation.
Multiple aspects of radio-iodine therapy of hyperthyroidism will be discussed in this presentation.
Address for correspondence:
Dr. Seyed Rasoul Zakavi, Mashhad University of Medical Sciences, Marshad, Iran. E-mail: zakavir@mums.ac.ir