system
Given the following post, respond as if you're commenting on Reddit:
human
How does radioactive iodine work?

For treatment of hyperthyroidism, how does the orally ingested radioactive iodine only have an effect on the thyroid? How does it not cross to other tissue/the rest of the body?
gpt
Your thyroid actively seeks to grab onto iodine in your body for its own processes.

The reason why iodine tablets are important to consume in the presence of a radiological release event is because it saturates your body with a (safe) amount of iodine such that when new (radioactive) external iodine enters your bloodstream, your thyroid doesn't pick it up because it's already "full". The result is that the radioactive version of the iodine is removed from your body as waste much sooner than it would have been, which means it spends less time irradiating your cells internally.

Radioactive iodine causes cellular damage to adjacent cells, by virtue of its radioactive nature. This can result in thyroid cancers.

Presumably intentionally ingested radioactive iodine treatment is done in dosages and circumstances that are intended to cause a specific targeted amount of damage to your thyroid, since it's going to collect there. Worst case you accidentally cause thyroid cancer (which you almost certainly will be on the watch for if you've ever had that treatment) which, if caught early enough, can be treated through a variety of means.