When is Radioactive Iodine used to treat Thyrotoxicosis?

Radioactive iodine is a very effective treatment for thyrotoxicosis; it works because the thyroid cells trap the radioactive iodine, which kills them. It can be used in all age groups and is even safe in children. Its role is limited in massive toxic goitres, particularly if there is a significant retrosternal element.Radioactive iodine does have an initial failure rate of 20% and therefore one in five patients needs a futher dose of radioactive iodine repeated.Most guidlines suggest stopping anti-thyroid drugs for 3 to seven day prior to giving radioactive iodine.The reasno for this is that radio-active iodine is more effective if antithyroid drugs are withdrawn. There is a down side to this practicein that a prolonged interval of antithyroid drug cessation is the risk of exacebation of the hyperthyroidism. Radio-active iodineis completely contraindicated in pregnancy and women should refrain from getting pregnant for at least 6 months after treatment. If the patient is breast-feeding it should be stopped before treatment and not restarted for 6 months.

Inadvertent Treatment with Radioiodine in early pregnancy.

It is mandatory for women of childbearing age undergoing radioactive iodine therapy that they have a pregnancy test prior to treatment. Unfortunately this is not always done and there have been reports of radioactive iodine administered during pregnancy. The management of the problem depends on the stage of the pregnancy.

Foetal uptake of radioactive iodine does not occur until 12 weeks into gestation. Radioactive iodine administered up to this time does not alter foetal thyroid function and is not an indication for termination of pregnancy. The situation is very different from 12 to 15 weeks.At this time in the pregnancy the foetal thyroid concentrates more radioactive iodine weight for weight than the maternal thyroid and there is a risk of hypothyroidism or cretinism in the neonate. There is very little data on this situation and there needs to be careful consultation describing the possible risks, and a frank discussion regarding the possible need for termination of the pregnancy. The management plan may be helped by dosimetry studies.

If the pregnancy is continuing to term and there is evidence of foetal hypothyroidism then intra-amniotic thyroxine should be considered. It is essential that the maternal thyroid levels are slightly elevated and that maternal hypothyroidism does not occur.

At delivery the neonate should be tested and if appropriate treated with thyroxine. The parents must be warned that treatment with thyroxine may be life long.

Use of radioactive iodine

Rather like the two ways of using antithyroid drugs, a similar philosophy exists with radioactive iodine. The most pragmatic treatment in our view is a single large dose of radioactive iodine, which destroys the entire thyroid necessitating early thyroid replacement. The second approach of repeated small doses of radioactive iodine is tiresome, and leads ultimately to thyroid replacement. Radioactive iodine is probably safe for patients with mild thyroid eye disease, but there is concern about its use and it is wise not touse it in patients with severe eye disease. Ifit is used in a patient with thyroid eye diseasesteroid cover is suggested for up to 4 weeks following treatment.

For reasonswe have never understood there is considerable objection to radioactive iodine treatment by United Kingdom patients.Our view is that their fears are unwarranted; similar fears do not exist in the USA. When surgery for thyrotoxicosis has failed radioactive iodine is the only remedy as re-do thyroidectomy with its high complication rate is rarely justified. An exception would be in pregnancy when surgery has failed, and subsequent antithyroid drugs have not controlled the toxicity.

There are studies looking at pre-treatment with oral lithium prior to radioiodine therapy. It has been shown toimprove the response to the radioiodine and dose for dose in large goitresto increase the incidence of cure. Interestingly it is not widely used.