Lucky for you I have access to medical references. After an extensive search this afternoon, during which I investigated thyroid scanning procedures and the various compounds used in them, I finally found the answer to your question.
The reason you are asked not to drink water is to prevent side effects from the radioactive iodine. Often thought to be without deleterious effect, it turns out that the stuff actually does have one. As it only occurs in well-hydrated subjects, patients are asked to refrain from drinking water.
I was a little surprised to find that this wasn’t listed in the possible complications of thyroid testing on all the patient-oriented websites I saw, but I suppose they don’t want to cause excessive worry. After all, it’s more important to get your thyroid fixed than to worry about a mere complication like this one, which is certainly not life-threatening. Still, there is all that about disclosure, but there really isn’t any other way to do this test. If people refused it, it would be a bad thing for them. I can understand that.
Anyway, I finally found this buried in old journal articles. Really, I had to dig to find this, but there it was. Reading through the articles–most of which I could only get in abstract form on such short notice, so I apologize for not being able to provide more complete information for you–it appears that the reason for the ban on water is, as I mentioned, to prevent a side-effect of the drug.
It seems that in the original studies conducted on this, which were longitudinal studies conducted over a number of years, so the cause-effect nature of this kind of got lost in translation, apparently, and this undoubtedly has made it easier for the drug companies to gloss over the risk, patients were tested in both hydrated and dessicated states to see the relative efficiency of uptake of the iodine. Patients were, of course, as you would in any good study, questioned carefully about their intake of water and, just so that forgetting and deliberate contrariness would not skew the results, but mostly to determine the length of time a patient would need to be dehydrated for best results, put on varying schedules of water consumption both before and after ingestion of the radioactive iodine. The problem turned out to be that male patients, particularly in early adulthood, who were not thoroughly dried out for a period of time–I forget what it was because I was interrupted constantly this afternoon–before and after taking the drug developed irreversible impotence. It took some time to appear–a year or two–so that it was difficult to connect the side effect with the ingestion of the drug and the degree of dehydration, but several data analyses have demonstrated that there is a cause and effect relationship, so it’s a done deal, so to speak. It looks like, as it so often does, that the testing methodology–patient instructions and so forth–was designed with this in mind, but that as time went on the original reasons were forgotten. So, you were told not to brush your teeth to prevent any early ingestion of water that could have had a disastrous consequence. Only, nobody really remembers what it is. Go figure.
As you know, of course, you won’t glow in the dark or anything from this, nor will you set off theft detectors in stores, and all that. But, I’m sure they told you this. So, as long as you didn’t drink any water, not even one or two minutes early, you’re safe.
