Radio-iodine treatment of hyperthyroidism

Radioactive iodine has been used successfully for almost 70 years since the first treatment took place at the Massachusetts General Hospital in Boston in 1941. However, it was not until after the Second World War that ¹³¹I became generally available for clinical applications (1). The radioactive iodine isotope is chemically identical to ‘stable’ iodine (¹²⁷I) and thus becomes a part of the intrathyroidal metabolism. Its principle of action is based on the emission of β-rays with a range of 0.5–2 mm in the tissue leading to high local radiation absorbed doses while sparing surrounding structures. The additional γ-ray component of ¹³¹I allows for scintigraphic imaging of the distribution in the gland and can also be used for pre- and post-therapeutic individual dosimetry (see below).

Several therapeutic options are available for the treatment of benign thyroid disorders, namely hyperthyroidism: surgical resection (hemithyroidectomy, near-total, or total thyroidectomy), long-term antithyroid drug medication (ATD), and radio-iodine therapy (RAIT) (2, 3). These different treatment modalities are used in varying frequencies depending on geographical location, e.g. iodine supply, availability and logistics, cultural background, and patient-specific features, e.g. goitre size, presence of local symptoms, age, and hormonal status. The diversity of approaches on an international scale still remains impressive and is reflected by a great heterogeneity throughout Europe and also when compared to the USA where radio-iodine therapy is still being applied more frequently than in most European countries (4–8).

Radio-iodine therapy was originally aimed at eliminating hyperthyroidism and thus leaving the patient euthyroid. Up-to-date strategies, however, established postradio-iodine induction of hypothyroidism as the treatment objective and, thus, it is included in the category of ‘cure’. This definition holds especially true for the management of Graves’ disease when long-term hypothyroidism was the rule and stabilization of euthyroidism failed in the majority of cases. In fact, the term ‘ablation’, meaning removal or destruction, has been increasingly used to characterize radio-iodine therapy and administration of larger amounts of radio-iodine have tended to make this a self-fulfilling prophecy. Although many clinicians prefer that the end result of treatment be the more easily managed hypothyroidism, others are still reluctant to give up the therapeutic ideal of euthyroidism as the preferred result of radio-iodine therapy and continue their efforts to solve the enigma of thyroid radiosensitivity.