Radioactive iodine is selectively absorbed by, and destroys, functioning thyroid cells. It is administered as a tasteless, colourless liquid. It is difficult to predict the precise dose necessary for euthyroidism and so the amount given represents a best guess. Typical doses vary from 5-15 mCi depending upon the size of the gland. Some authorities recommend a very large dose to induce rapid hypothyroidism which is then corrected with thyroxine. A response is not detectable for 6-8 weeks during which time cover with carbimazole or a beta blocker is required. Carbimazole must be delayed for 2-3 days after taking radioactive iodine as it prevents iodine uptake.
Hyperthyroidism resolves in up to 60% of patients after a single dose of I-131. A second dose should be given if there is no improvement within about 4 months. About 8% per annum become hypothyroid in the first few years; permanent hypothyroidism is present in 80% by 20 years.
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