How to Prepare for Blood Work: Fasting, Biotin, Timing and the Rest

Biotin — vitamin B7, sold for hair, skin and nails, and present in many "beauty" and B-complex supplements — interferes with the chemistry many hormone assays use. It can make thyroid results look like hyperthyroidism, push troponin falsely low, and distort testosterone, estradiol, cortisol, vitamin D and others in either direction depending on the assay. The FDA issued a safety communication about it in 2017 and reinforced it in 2019 after a death attributed to a falsely normal troponin.

The interference is dose-dependent and the "hair and nails" supplements are the high-strength ones. Stop biotin-containing supplements for at least two to three days before the draw — a week if the label says it is for hair — and tell us if you take one at all, because a strange thyroid or hormone result is far more often biotin than disease.

Several hormones follow a daily rhythm, and the reference ranges assume a morning sample. Testosterone peaks early and falls through the day — a man's afternoon result can be low enough to earn a diagnosis he does not have, which is why the Endocrine Society requires a morning draw, repeated. Cortisol is highest on waking and falls steeply; an afternoon cortisol is a different test. Iron and some thyroid values move too.

For a woman still cycling, which day matters as much as which hour for estradiol and progesterone — our page on when to test hormones in perimenopause covers it.

Intense exercise — a long run, a heavy lifting session, a race — raises creatine kinase, liver enzymes, creatinine and inflammatory markers for a day or two afterwards, and can transiently move testosterone and cortisol. A panel drawn the morning after a hard session can show "liver damage" that is a sore quadriceps. Light activity is fine. Save the big session for after the draw.

Alcohol raises triglycerides, liver enzymes and the GGT that flags it, and disturbs sleep, which moves cortisol and glucose. Two or three days off before a draw gives a cleaner baseline — and the alcohol and your markers page explains what a drinking pattern does to the panel over the longer term.

Dehydration concentrates the blood and makes kidney markers and hemoglobin look worse than they are, and it makes the draw harder. Drink water normally the evening before and the morning of.

Take your usual prescription medications unless we have specifically said otherwise — there are a few exceptions, like thyroid medication before a thyroid panel or the timing of testosterone for a man on replacement, and we will tell you. Do not stop anything on your own initiative to "get a true reading"; the reading we want is the one on your treatment, and stopping some medications abruptly is dangerous.

Supplements are different: biotin is the important one, above, and it is reasonable to skip the rest that morning.

Water only from the evening before. No biotin for several days. Morning draw. No hard training the day before. Light on alcohol for a few days. Usual medications, unless told otherwise. Tell us everything you take.