131I is treated by the body just like the ordinary nonradioactive isotope, so that when swallowed it is concentrated in the thyroid gland. It emits mainly (5 radiation (90%), which penetrates only 0.5 mm of tissue and thus allows therapeutic effects on the thyroid without damage to the surrounding structures, particularly the parathyroids. It also emits
2 The newer preparations approximately triple the cost of diagnostic investigations requiring contrast media. With a fatality rate of -1/50 000 in patients receiving the older agents, hospitals are faced with an interesting cost-benefit equation.
some gamma rays, which are more penetrating and can be detected with a radiation counter. 131I has a physical (radioactive) t'/2 of 8 days.
131I is the preferred initial treatment for hyperthyroidism caused by Graves' disease in North America. It is contraindicated in children and pregnant or breast-feeding women, and can induce or worsen ophthalmopathy. It is used in combination with surgery in some cases of thyroid carcinoma, especially those in which metastases are sufficiently differentiated to take up iodide selectively.
In hyperthyroidism the beneficial effects of a single dose may be felt in one month, and patients should be reviewed at 6 weeks to monitor for onset of hypothyroidism. The maximal effect of radioiodine may take 3 months, ^-adrenoceptor blockade and, in severe cases, an antithyroid drug (but see footnote 1) will be needed to render the patient comfortable whilst waiting; this is more likely when radioiodine is used for treatment of patients with relapsing thyrotoxicosis. Very rarely radiation thyroiditis causes excessive release of hormone and thyroid storm. Repeated doses are sometimes needed.
The adverse effects of radioiodine are as for iodism, above. In the event of inadvertent overdose, large doses of sodium or potassium iodide should be given to compete with the radioiodine for thyroid uptake and to hasten excretion by increasing iodide turnover (increased fluid intake and a diuretic are adjuvants).
Radioiodine offers the advantages that treatment is simple and carries no immediate mortality. However, it is slow in acting and it is difficult to judge the dose that will render the patient euthyroid.
In the first year after treatment 20% of patients will become hypothyroid. After this 5% of patients become hypothyroid annually, perhaps because the capacity of thyroid cells to divide is permanently abolished so that cell renewal ceases. Patients must therefore be followed up indefinitely after radioiodine treatment, for most are likely to need treatment for hypothyroidism eventually. Because such follow-up over years may fail and because the onset of hypothyroidism may be insidious and not easily recongnised, some physicians prefer deliberately to render patients hypothyroid with the first dose and to educate them on the use of replacement therapy which is safe and effective.
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