@roch
I discovered this a few years ago when a neurologist was testing me in reaction to what chemo had done to me. When my PCP saw the test results (way above the upper range for thyroid), he immediately referred me to an endocrinologist and for a thyroid ultrasound. For whatever reason, I personally could not believe I was hyperthyroid. I began researching on the net and came across articles from 2016 which noted that biotin had that effect on blood assays; (the oncologist had recommended 50,000 mcg. of biotin to help regrow my lost hair and nails); and that, in the past, endocrinologists unknowingly had been treating biotin-takers for hyperthyroidism. We decided to see if that was the culprit and I was retested after having gone off the biotin for 3-4 days. And, yes, the blood assay came back as normal. Ever since, I have stopped all supplements a few days before blood tests, wondering what else might effect them and just hadn't been discovered, yet. Incidentally, and surprisingly, most doctors are unaware of this until you bring the FDA warning to their attention.