I can confirm that biotin will affect thyroid tests. I take it due to thinning hair and poor nail quality and I don't actually think it's helped my hair or nails at all (ha ha) but it did increase my TSH level in a recent blood test by a few points. My doctor didn't request I quit taking it for a few days and it caused some confusion, so ...........yes, quit taking Biotin for a few days before a blood draw for TSH.

Biotin can mess up thyroid tests in unexpected ways... it can make the results read extreme at either end of the “normal” range. It happened to me. I didn’t know about that possibility before my thyroid test went askew, but I then researched possible causes and figured it out. My PCP was ready to start taking more drastic measures to address my “thyroid problem”— but she never asked about biotin. I brought it to her attention after my research. Subsequent testing after I quit the biotin has been normal. My hair was starting to be “okay” after chemo, but then the anastrozole thinned it greatly. (I was taking Biotin to try to thicken it.) I have been on anastrozole for four years.