Radioiodine (131I) therapy is indicated in patients with nearly all causes of hyperthyroidism. It may safely be given to patients of all age groups but is less often given to children under 10 years old. It is completely contraindicated in pregnancy and while breast feeding, but there is no increased risk of thyroid cancer, leukaemia or solid tumours. Administration of radioiodine must conform to regulations and definitions laid down by ARSAC And POPUMET. Medical staff authorising therapy must hold an ARSAC licence. The recommended strategy is to give an activity sufficient to render the patient rapidly euthyroid and maintain that state or achieve no more than a low rate of hypothyroidism in subsequent years. A range of activity (300-800 MBq) is suggested depending on the clinical state. Antithyroid drugs may be given before or after (or both) radioiodine if necessary. Full written information should be given to the patient and written consent obtained. A structured follow-up should be used ensuring regular measurement of TSH or FT4. Close cooperation with the patient's general practitioner is recommended throughout the assessment, treatment and follow-up. Shared care with a computer based follow-up system is recommended.