Question: Can sulforaphane hurt the thyroid?
What I can say is that sulforaphane does generate thiocyanate ions, which do inhibit uptake of iodine into the thyroid and mammary glands. Although that is a matter of the ratio between isothiocyanate, or between thiocyanate ions and iodine. And so in principle, most uses of sulforaphane, in the context of adequate iodine shouldn't be an issue. I believe at some dose you're going to run into a problem with balancing with iodine, especially in people who have marginal iodine status, but I don't have any studies to back up what point that is. But I have seen cases of people where they got brain fog when they were taking sulforaphane and it went away when they took iodine.
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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.