From the Mayo Clinic:
Radioactive iodine treatment uses large doses of a form of iodine that's radioactive. Radioactive iodine treatment is often used after thyroidectomy to kill any remaining healthy thyroid tissue, as well as microscopic areas of thyroid cancer that weren't removed during surgery. Radioactive iodine treatment may also be used to treat thyroid cancer that recurs after treatment or that spreads to other areas of the body.
Radioactive iodine treatment comes as a capsule or liquid that you swallow. The radioactive iodine is taken up primarily by thyroid cells and thyroid cancer cells, so there's a low risk of harming other cells in your body.
Side effects may include:
- Dry mouth
- Dry eyes
- Altered sense of taste or smell
- Pain where thyroid cancer cells have spread, such as the neck or chest
Most of the radioactive iodine leaves your body in your urine in the first few days after treatment. During that time you'll need to take precautions to protect other people from the radiation. For instance, you may be asked to temporarily avoid close contact with other people, especially children and pregnant women.
From Endocrine Web:
Thyroid cells are unique in that they have the cellular mechanism to absorb iodine. The iodine is used by thyroid cells to make thyroid hormone. No other cell in the body can absorb or concentrate iodine. Physicians can take advantage of this fact and give radioactive iodine to patients with thyroid cancer. There are several types of radioactive iodine, with one type being toxic to cells. Papillary cancer cells absorb iodine and therefore they can be targeted for death by giving the toxic isotope (I-131). Once again, not everybody with papillary thyroid cancer needs this therapy, but those with larger tumors, spread to lymph nodes or other areas, tumors which appear aggressive microscopically, and older patients may benefit from this therapy. This is extremely individualized and no recommendations are being made here or elsewhere on this web site...too many variables are involved. But, this is an extremely effective type of "chemotherapy" will little or no potential down-sides (no hair loss, nausea, weight loss, etc.).
Uptake is enhanced by high TSH levels; thus patients should be off of thyroid replacement and on a low iodine diet for at least one to two weeks prior to therapy. It is usually given 6 weeks post surgery (this is variable) can be repeated every 6 months if necessary (within certain dose limits).
There's also some information WebMD, which tells you what to expect:
Radioactive iodine, given in a liquid form, is absorbed and concentrated by the thyroid gland. The treatment destroys thyroid tissue but does not harm other tissue in the body.
- See an illustration of the thyroid gland .
While radiation can cause thyroid cancer, treatment of hyperthyroidism with radioactive iodine does not increase your chances of getting thyroid cancer.
What To Expect After Treatment
Within days, the radioactive iodine passes out of your body in your urine.
To avoid exposing other people to radioactivity, it is important to do the following for the first 5 days after your treatment:
- Drink plenty of fluids.
- Avoid spending a lot of time around others, especially children and pregnant women.
- Do not sit next to someone in a motor vehicle for more than 1 hour.
- Avoid kissing or sexual intercourse.
- Sleep in your own room.
- Use separate towels, washcloths, and sheets. Wash these and your personal clothing separately for 1 week.
To further reduce the chance of exposing other people to radioactivity:
- Wash your hands with soap and lots of water each time you use the toilet.
- Keep the toilet very clean. Men should urinate sitting down to avoid splashing. Also, flush the toilet 2 or 3 times after each use.
- Rinse the bathroom sink and tub thoroughly after using them.
- Use separate (or disposable) eating utensils for the first few days and wash them separately.
Why It Is Done
Radioactive iodine may be used to treat hyperthyroidism in people who have noncancerous (benign) thyroid nodules that make too much thyroid hormone.
Radioactive iodine is also used if you have your thyroid removed (thyroidectomy) because of thyroid cancer. Radioactive iodine therapy destroys any remaining thyroid tissue or cancer cells that were not removed during surgery.
How Well It Works
In almost all cases, your thyroid hormone levels will return to normal or below normal after radioactive iodine treatment. This may take 8 to 12 weeks or longer. If your thyroid hormone level does not go down after 6 months, you may need another dose of radioactive iodine.
If you have thyroid cancer and you are treated with radioactive iodine, it may take from several weeks to many months for your body to get rid of any remaining cancer cells.
Your thyroid nodule is unlikely to grow after being treated with radioactive iodine.
The risks from radioactive iodine treatment include: