Background: Radioiodine therapy using I131 is a well established, safe, cheap and effective treatment for thyrotoxicosis. Current recommendations support the administration of 400-550 Mbq. At this centre, we use a dose of 550 Mbq with the aim of minimising the chance of the patient remaining thyrotoxic following therapy.
Aims: To determine (a) the thyroid status of patients following treatment with I131 at a dose of 550 Mbq, (b) the speed of onset of hypothyroidism, and (c) the influence of various baseline parameters on the outcome.
Methods: Retrospective audit of patients receiving radioiodine treatment for thyrotoxicosis between 2002 and 2006 at our centre.
Results: Records from 92 patients (75% female), median age 55 years (range 1889) were examined. Median radioiodine dose 527 Mbq (range 440560); median duration of first follow-up was 7.5 weeks (range 224 weeks). 30(32.6%) had a diagnosis of Graves disease, 34(37%) toxic multinodular goitre and in the remaining 28 (20.4%) the underlying aetiology was unclear. After radioiodine therapy 11(12%) were euthyroid, 1 remained hyperthyroid and was lost to follow up, and 5(5.4%) patients remained thyrotoxic and required a second dose of radioiodine. Of these 5 patients 4 had a diagnosis of Graves disease and 1 had undefined thyrotoxicosis. The remaining 75(81.5%) became hypothyroid. The majority(70%) were diagnosed with hypothyroidism in the first 16 weeks after therapy, of whom 34% were hypothyroid by 8 weeks and a further 21% by 12 weeks. Patients who remained thyrotoxic following radioiodine had significantly higher FT4 at diagnosis than those who were euthyroid or hypothyroid after therapy (74.8 pmol/L vs. 39.7 pmol/L vs. 38 pmol/L P<0.001). There was no significant effect of age,sex or prior medical treatment on the outcome.
Conclusion: Radioiodine administered at a dose of 550 Mbq is effective for the treatment of thyrotoxicosis. Hypothyroidism is common and in the majority of cases occurs within the first 16 weeks of administration of radioiodine.