Can a toilet be a significant source of radon?

A question to BRAWM or anyone that might be willing to take a swing at this:

I understand that toilets and other porcelain fixtures can be mildly radioactive and that the impurities are similar to those that can be found in household construction materials (uranium, thorium...) so I imagine a toilet could emit some radon. I see bricks frequently mentioned as a possible culprit in elevated home radiation levels, but not porcelain fixtures. Is that because the radioactivity in toilets, etc. is so low that their radon contribution is almost certainly negligible?

How radioactive is an average toilet?