Biotin and blood tests: false thyroid results
A typical story goes like this. Someone takes a hair, skin and nails supplement for a few months. A routine thyroid test comes back with a low TSH and a high free T4, the classic pattern of an overactive thyroid. More tests get ordered, maybe a scan, and then a later test with the supplement stopped is completely normal.
The cause is biotin, also called vitamin B7 or vitamin H. Many lab tests use biotin as part of the chemistry that captures the hormone being measured. When your blood carries a lot of biotin, it gets in the way, and depending on how the test is built the result reads falsely high or falsely low. The US FDA warned doctors and patients about it in 2017. Nothing is wrong with the thyroid or the hormone. The number is wrong.
Why biotin upsets some lab tests
Many automated tests rely on a strong natural bond between biotin and a protein called streptavidin. The lab's reagents carry biotin, the test surface carries streptavidin, and the bond holds the hormone in place long enough to measure it.
Biotin from a supplement competes for the same streptavidin. What happens next depends on the design of the test:
| Test design | Effect of extra biotin | Examples |
|---|---|---|
| Sandwich tests, for larger molecules | Falsely low | TSH, LH, FSH, PSA, hCG, troponin, PTH |
| Competitive tests, for smaller molecules | Falsely high | Free T4, free T3, testosterone, estradiol, cortisol, DHEA-S, vitamin D |
Which of your tests are affected depends on the lab's machines. Not every lab uses biotin-based tests for every marker, and several manufacturers have changed their tests to tolerate more biotin. You can't usually tell from the report.
Which supplements contain enough biotin to matter
The daily requirement for biotin is tiny, and the amount in an ordinary multivitamin is small. The problem is products sold for hair, skin and nails, which often contain many hundreds of times the daily requirement in one tablet. Some products sold for other reasons contain a lot too. Read the label: biotin is often listed in micrograms, and the large numbers are the ones to notice.
What a biotin-affected thyroid result looks like
This is TSH and free T4 from one person across 13 tests over two years:
Sample data, not a real person.
One draw stands out. TSH dropped and free T4 jumped, which on its own looks like an overactive thyroid. It was taken while she was on a high-strength hair supplement. The next test, without it, picks up exactly where the line was heading: TSH still creeping up, free T4 still sliding down. On a single report, that one draw would have pointed the wrong way. On a timeline, it's obviously out of line with everything around it.
How to avoid a biotin-affected result
Tell whoever orders the test what you take
List supplements as well as medicines. Biotin is in many products that don't say "biotin" on the front, including some hair, skin and nail blends and multivitamins.
Stop it before the test, if your doctor agrees
How long depends on how much you take and on the lab's tests. Labs often ask for at least a day or two, and longer for high-strength products, since biotin can take longer to clear in some people. Ask the lab or your doctor for their advice. If you can't stop it, tell the lab, since some can rerun a test in a way that removes biotin.
Don't restart it until after the draw
Taking it the morning of the test is the worst timing. Wait until after the sample is taken.
Repeat a surprising result before acting on it
A thyroid or hormone result that doesn't match how you feel, or doesn't fit your earlier results, is worth repeating with biotin stopped. Ask your doctor whether a repeat makes sense before more tests or treatment.
Write it down next to the result
A note saying "on biotin" next to a result explains a strange number years later, when nobody remembers the supplement.
Keep the other conditions the same too
Same lab, same time of day, same fasting. Our post on fasting before a blood test covers the rest of the routine.
Which results to be most careful with
Thyroid tests are the best-known example, because a low TSH with a high free T4 looks just like an overactive thyroid. Our post on the thyroid panel explains how TSH and free T4 normally move together, which is what makes the biotin pattern stand out.
Hormone panels can be affected too. Testosterone and estradiol can read high while LH and FSH read low, which looks like the pattern on testosterone treatment. Our posts on the testosterone blood test and LH and FSH cover what those normally show.
The most serious case is troponin, a heart test used in emergencies, which biotin can make read falsely low. If you take high-strength biotin and end up in an emergency department with chest pain, tell the staff.
A record you can keep by hand
| Date | Lab | Time | Biotin? | TSH (mIU/L) | Free T4 (ng/dL) | Notes |
|---|---|---|---|---|---|---|
| 2025-01-25 | City Lab | 08:10 | no | 2.9 | 1.13 | |
| 2025-03-22 | City Lab | 08:05 | yes, daily | 0.9 | 1.86 | hair supplement since January |
| 2025-05-17 | City Lab | 08:15 | stopped 5 days before | 3.8 | 1.01 |
Add the biotin column even if you don't take any. It's the question a doctor will ask about any odd result.
Where dialedin fits
We built dialedin so a single odd result is seen next to the ones around it.
You upload a lab PDF and it is read into individual markers, which you check before saving. Every report goes onto one timeline, so an out-of-line result like the one above is easy to spot, and the same marker under different lab names stays one line. You can record what you were taking and when, supplements included, and add an optional daily check-in to see which levels line up with your better days. You can export your history as a PDF for your doctor.
It's free while we're in beta. If your results are spread across reports, put them on one timeline and see which ones don't fit.
This article is general information, not medical advice, and it can't take your own history into account. Talk to your doctor before stopping or starting any supplement, and about what your results mean.