Chris Kresser: Let’s talk about iodine first. I do test for iodine, but I do it based on diet. If the patient is not consuming sea vegetables or dairy products or fish, particularly high-iodine species of fish, then I would suspect iodine deficiency, especially if they have hypothyroidism, and I would test using … generally I do either a 24-hour urine or spot urine rather than serum for iodine.
Then for selenium, if they’re eating ocean fish, then selenium deficiency is pretty rare because 16 of the 25 highest sources of selenium are ocean fish. If they’re eating Brazil nuts, it’s going to be very unlikely. But if they’re not, then testing red blood cell selenium might be a good idea. Toenail selenium is actually what’s used in studies as an indicator of long-term selenium status, but I haven’t found a commercial lab that does it yet. That would be better because selenium can change quite a bit in the short term, and the toenail status is a better indicator of longer-term selenium intake.
Given the narrow therapeutic range for both of these nutrients, this is why I prefer food to supplements. The body has, I think, more mechanisms for regulating intake through food than it does through supplements.