Re: radon in water
Possibly consider the chlorine & maybe fluoride in your water, if on a municipal system? Both of these chemicals are damaging to thyroid. Chlorine is easy to remove with inexpensive, readily available carbon-block filters (pitchers or faucet-attached). Fluoride is more difficult to eliminate, pretty much requires spendy reverse osmosis.
Uptake of fluorine & chlorine during showers & baths can be equal to or greater than that taken in via drinking water. Both chemicals are absorbed by the lung (steamy vapors) as well as through the skin. Shower filters to remove chlorine are available.
However, I would choose not to use radon-contaminated water for personal use. There may be more at stake than the thyroid gland.