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What to tell patients about radioiodine therapy

BMJ 2006; 333 doi: (Published 03 August 2006) Cite this as: BMJ 2006;333:271
  1. Daniel J Cuthbertson (d.j.r.cuthbertson{at}, clinical lecturer in diabetes and endocrinology,
  2. John Davidson, consultant in nuclear and general medicine
  1. Ninewells Hospital and Medical School, Dundee DD1 9SY
  2. Ninewells Hospital and Medical School, Dundee DD1 9SY

    Avoid close contact with other people, don't try to conceive, and take your radiation certificate when flying

    In this week's BMJ Gangopadhyay and colleagues report the case of an unfortunate patient who activated an airport radiation detector six weeks after receiving radioiodine 131I therapy and was consequently subjected to intense examination and interrogation (p 293).1 No clinical practice guidelines cover this type of problem. So, on current evidence, how should doctors advise patients who are about to receive radioiodine therapy?

    Each year radioiodine is administered to 10 000 people for hyperthyroidism in the UK.2 After injection or ingestion, radioiodine initially concentrates in the thyroid gland and then circulates through the bloodstream, with 80% excreted renally. While the radioactive urine is stored in the bladder, until it is voided, the gonads are irradiated: after an administered dose of 750 MBq of 131I the testes receive a radiation dose of 17-19 mGy and the ovaries 28-31 mGy.3

    To reduce the risk of any radioactive germ cells causing irradiation of the fetus, the Department of Health Administrations of Radioactive Substances Advisory Committee (ARSAC) recommends that male and female patients should be advised to avoid conception for four months after treatment.4 The American Thyroid Association suggests a period of 6-12 months.5 Also, although currently no evidence shows that treatment is associated with an increased incidence of infertility in women,6 men should be warned that they may have a reduced sperm count and temporary infertility for up to two years after therapy.5 We can reassure patients about future conception, as there is no association between the calculated internal radiation dose and risk of genetic abnormalities observed in subsequent pregnancies.6 7

    Patients with young children at home should be advised to avoid prolonged daily close contact (within less than 1 metre) with them for a time dependent on the dose of radioiodine to be administered. The inconvenience or impracticality of this requirement often deters many patients from having treatment.

    Long term follow-up data show no increased risk of cancer after radioiodine therapy and, contrary to some patients' misconception, there is no overall excess mortality from cancer.8 9 Some studies have highlighted a small increased risk of thyroid cancer, but the increased absolute risk is very small—in about 7500 patients treated with radioiodine nine cases of thyroid cancer were observed compared with 2.8 expected.8 10

    The only recognised long term adverse effect of radioiodine therapy is hypothyroidism, so all patients are warned that they might need lifelong thyroxine replacement. The rate at which hypothyroidism develops depends on the dose of radioiodine. Current evidence says that best practice is to give higher doses of 131I to render patients hypothyroid more quickly, given the observed increased all cause mortality in hyperthyroid patients treated with radioiodine (in about 105 000 person years of follow-up, 3611 deaths were observed compared with 3186 expected).11 This increased mortality is no longer seen, however, once patients are hypothyroid and receiving replacement therapy.12

    Patients are advised to avoid public transport for two weeks so that they do not expose nearby passengers to radioactivity but, to date, advice regarding air travel and radiation detectors has been overlooked in many centres. It is not possible to devise prescriptive guidelines to clearly define the period during which patients may activate security alarms at airports. Despite the constant biological half life of 131I (eight days), its effective half life varies widely with the patient's renal function. Furthermore, anecdotes of patients who have travelled to eastern Europe without incident after radioiodine therapy, only to activate a security alarm on entering the United States two weeks later, suggest that different detectors are set at different radioactive thresholds. Not surprisingly, the British Airports Authority was unwilling to give any information regarding the sensitivity of their detectors when we approached it for clarification.

    The only practical solution must be to adequately counsel anyone receiving radioiodine (or indeed any radioisotope) about travelling overseas and advise them to carry their radiation certificates. To prevent the alarm bells ringing, this practical advice should be incorporated into the new guidelines on the use of radioiodine for thyroid disease that are to be published later this year by the Royal College of Physicians.


    • Competing interests None declared.

    • Practice p 293


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