Biotin and blood tests

Your doctor ordered a thyroid test because your hair is falling out. The result comes back odd: a very low TSH, a slightly high T4, even though you feel the same as ever. On your bedside table sits a jar of “hair, skin and nails” gummies. Your thyroid may not be the one talking. It could be the biotin.

Biotin, also called vitamin B7, is the star of hair supplements. It has a side effect few people know about: it can skew some blood tests, and nothing on the lab report will flag it.

Why biotin throws off lab tests

Many modern blood tests are immunoassays. To catch the substance they measure, some of them rely on biotin, usually paired with a protein called streptavidin that grips it very tightly³. Think of a magnet and its target. If your blood already holds a lot of biotin, it gets in the way and the signal is distorted.

Which way the error goes depends on how the test is built. In a study published in JAMA, “competitive” tests mostly gave falsely high results, while “sandwich” tests gave falsely low ones⁵. For you, that means one thing: the same supplement can push one result up and another down.

Which tests are affected

The list is long. The French medicines agency (ANSM) mentions tests for hormones, tumour markers, specific proteins and viral serology, mostly on automated lab analysers, plus at least one home test³. It surveyed the manufacturers.

ANSM, survey of reagent manufacturers³

Two groups of tests come up most often. First, the thyroid: TSH, T3, T4 and thyroglobulin¹. Then the heart. The US Food and Drug Administration (FDA) has received reports of falsely low troponin results caused by biotin⁴. Troponin is the marker A&E teams check when someone has chest pain: a falsely reassuring result can mean a missed heart attack.

US lab guidance adds parathyroid hormone, oestradiol, testosterone, progesterone, ferritin, vitamin B12, PSA, LH and FSH⁶. Several of these are exactly the tests ordered to look for a deficiency or a hormone imbalance when hair is falling out.

What a study of six volunteers found

The clearest study was run in the United States on six healthy adults. For one week they took 10 mg of biotin a day, 333 times the 30 µg recommended in the diet⁵. That isn’t extreme: some over-the-counter supplements contain up to 100 mg per dose⁶. Their blood was tested beforehand, on day seven about two hours after the last dose, and again a week after stopping.

Of 37 tests run on four different analysers, 23 used biotin. Nine of those 23 gave skewed results; none of the other 14 did⁵. The biggest shift was in TSH on one analyser: it fell by 94% on average, and every result dropped below the normal range. On another system, T3 and free T4 went up instead⁵. A week after stopping, everything was back to normal⁵.

The limits are clear, and the authors state them: six people, no placebo group, healthy participants, and no measure of the smallest dose that can skew a test⁵. The study shows the problem is real. It doesn’t tell you where it starts for you.

The thyroid trap

A thyroid that isn’t working properly can make hair fall out, which is why the test is ordered so often (we explain this in our article on the thyroid and hair). With biotin, the result can wrongly suggest an underactive or overactive thyroid. The ANSM warns of a risk of misdiagnosis and inappropriate treatment¹.

In its letter to doctors, the agency calls this risk high, given the growing use of high-dose supplements. It singles out people whose thyroid treatment has to be adjusted precisely: pregnant women, children, older people, and people followed for thyroid cancer². In most reported cases, results went back to normal once biotin was stopped or another test method was used².

On a thyroid forum, one patient describes finding out about the problem years later⁷:

Posted on the Thyroid UK forum⁷.

How long to wait before a blood test

This is the question everyone asks, and French health authorities don’t give a single figure: the wait varies with the test. The most precise guidance comes from the American Association for Clinical Chemistry, published in 2020⁶.

Why the gap? At low doses, biotin clears from the blood quickly, with a half-life of about 2 hours. At high doses, measurements have found up to 18.8 hours⁶. The kidneys do the clearing: when they work less well, biotin stays around longer⁶.

On forums you’ll often read “stop 7 days before any blood test”. That’s patient advice, not an official rule. It does cover a wide margin, and the JAMA study did find results back to normal a week after stopping⁵. If biotin was prescribed to you as a medicine, ask your doctor before pausing it.

What to tell the lab

The ANSM’s rule is simple: tell your doctor and the lab if you take biotin or have taken it recently. The lab can then choose a different reagent or method²³. A few habits help:

• Turn the jar over and read the ingredients: biotin is also called vitamin B7, or B8 in France, and often hides in a “skin and nails” supplement or a multivitamin.
• Add it up: two products taken together may each contain some. What’s in a hair supplement lists the ingredients that keep turning up.
• Note the date and time of your last dose, and mention it when your blood is taken.
• For a home test, check the leaflet doesn’t warn about biotin interference³.
• In A&E with chest pain, say so too: that’s where the stakes are highest⁴.

If a result doesn’t match how you feel, your doctor can consider this interference. The ANSM explicitly asks them to². To prepare for the appointment, the questions to ask your doctor will help you cover everything.

Taking a break without losing track

Stopping biotin for a few days before a test often worries people: what if their hair suffers? A few days’ pause won’t change what you see in the mirror, because a hair grows over months. The real risk is forgetting when you stopped, when you started again, and exactly what you were taking.

In Calvap, the journal keeps the start, pause and restart dates of every product. On the day of your blood test, you have the answer to hand. And your comparable photos carry on every week, on the same timeline: when you go back to your routine, you’ll see over several months whether this supplement really changes anything for your hair.