Can taking 10,000 mcg of biotin everyday cause thyroid levels to be high in one test and low in another?

No; thyroiditis can!: Biotin is a benign vitamin with no known toxicity at any dose and is not known to affect thyroid. 10,000 mcg sounds like a lot but if you call it 10 mg it does not seem that high. See http://lpi.oregonstate.edu/mic/vitamins/biotin Fluctuating thyroid levels are classic in thyroiditis; get tested for anti-thyroglobulin and anti-thyroperoxidase (TPO) antibodies. See http://tinyurl.com/gobuw4x