Radioactive Iodine

RADIOACTIVE IODINE

The stable isotope of iodine is ¹²⁷I. Its radioactive isotopes of medicinal importance are:

131 I: physical half-life is 8 days—most commonly used in medicine.

123I: physical half-life is 13 hours—only rarely used diagnostically.

125I: physical half-life is 60 days.

Their chemical behaviour is similar to the stable isotope.

131I emits X-rays as well as β particles. The former are useful in tracer studies, as they traverse the tissues and can be monitored by a counter, while the latter are utilized for their destructive effect on thyroid cells. 131I is concentrated by thyroid, incorporated in colloid—emits radiation from within the follicles. The β particles penetrate only 0.5–2 mm of tissue. The thyroid follicular cells are affected from within, undergo pyknosis and necrosis followed by fibrosis when a sufficiently large dose has been administered, without damage to neighbouring tissues. With carefully selected doses, it is possible to achieve partial ablation of thyroid.

It is used as sodium salt of 131I dissolved in water and taken orally.

Diagnostic

25–100 μ curie is given; counting or scanning is done at intervals. No damage to thyroid cells occurs at this dose.

Therapeutic

The most common indication is hyperthyroidism due to Graves’ disease or toxic nodular goiter. The average therapeutic dose is 3–6 m curie—calculated on the basis of previous tracer studies and thyroid size. Higher doses are generally required for toxic multinodular goiter than for Graves’ disease. The response is slow— starts after 2 weeks and gradually increases, reaching peak at 3 months or so. Thyroid status is evaluated after 3 months, and a repeat dose, if needed, is given. About 20–40% patients require one or more repeat doses.

Advantages

·   Treatment with 131I is simple, conveniently given on outpatient basis and inexpensive.

·      No surgical risk, scar or injury to parathyroids/recurrent laryngeal nerves.

·         Once hyperthyroidism is controlled, cure is permanent.

Disadvantages

·      Hypothyroidism: About 5–10% patients of Graves’ disease treated with 131I become hypothyroid every year (upto 50% or more patients may ultimately require supplemental thyroxine treatment). This probably reflects the natural history of Graves’ disease, because only few patients of toxic nodular goiter treated with 131I develop hypothyroidism. Moreover, eventual hypothyroidism is a complication of subtotal thyroidectomy/prolonged carbimazole therapy as well.

·        Long latent period of response.

·     Contraindicated during pregnancy—foetal thyroid will also be destroyed resulting in cretinism, other abnormalities if given during first trimester.

·        Not suitable for young patients: they are more likely to develop hypothyroidism later and would then require lifelong T4 treatment. Genetic damage/cancer is also feared, though there is no evidence for it.

131I is the treatment of choice after 25 years of age and if CHF, angina or any other contraindication to surgery is present.

Metastatic carcinoma of thyroid (especially papillary or those cases of follicular which concentrate iodine), 131I may be used as palliative therapy after thyroidectomy. Much higher doses are required and prior stimulation with TSH is recommended.