Question: How do I know if biotin deficiency is causing my hair loss and fungal rashes?
The plasma or serum biotin level is not a good marker of biotin status because it's not very sensitive. And so, if you're in the bottom 10% of it, I would definitely take that as a red flag that you might have biotin deficiency, the best test for biotin, well, the gold standard for biotin deficiency is beta-hydroxyisovalerate in the urine after a leucine challenge. I don't know how you'd get anyone to give you that, but the next best thing is to get a urinary organic acid test that has as beta-Hydroxyisovaleric acid on it and I know that the Genova ION panel, which is the one I usually prefer has that, but a lot of other ones do too. So I, if you get one, I would just check and see whether the beta-hydroxyisovalerate is on the list of analytes.
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DISCLAIMER: I have a PhD in Nutritional Sciences and my expertise is in performing and evaluating nutritional research. I am not a medical doctor and nothing herein is medical advice.