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. If you would like to be part of the next live Ask Me Anything About Nutrition, sign up for the CMJ Masterpass, which includes access to these live Zoom sessions, a private discussion group, premium features on all my content, and hundreds of dollars of exclusive discounts. You can sign up at https://chrismasterjohnphd.com/masterpass/ and use the code QANDA to get 10% off the membership for life. From now through March, I will be working full-time on finishing my Vitamins and Minerals 101 book, while reserving a portion of my time for consulting clients. You can pre-order my book at https://chrismasterjohnphd.com/book. You can sign up for a consultation at https://chrismasterjohnphd.com/consultations. 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.