Just to back up what others have posted about biotin affecting thyroid testing. I had a thyroid blood test done at GP unexpectedly and forgot that I should have stopped my supplements a few days before. I was just supplementing with Thornes b complex so not a high dose. The test came back and my levels had all suddenly normalised.
I asked for it to be redone as it was so anomalous and I had been supplementing. The test was redone about a week later and my TSH had shot up and my T4 down. I now have a diagnosis and medication!
Definitely stop supplementing biotin 4/5 days before any thyroid testing.