Hi SELF Family,

You bought it for your hair. Your next blood test may notice it, too.

Biotin is a B vitamin found in many hair-and-nail pills. Here is the part the bottle may not make clear: a high dose can make some blood test results look wrong. [1]

In Friday’s supplement video, we looked at what your kidneys must clear. Today, I want to add a different question: Could a pill change how the lab reads your blood?

That is what researchers tested in a small study published in JAMA. Six healthy adults took 10 milligrams of biotin each day for one week. That dose is sold in some supplements. On a label, it may read 10,000 mcg.

The researchers checked several kinds of lab tests. Of the 23 test methods that used biotin as part of the process, nine showed interference. Some readings went up when they should not have. Others went down. None of the 14 methods that did not use biotin showed this problem. [2]

Six people is a small study. It cannot tell us how often this happens in everyday care. But it shows why the lab’s test method matters.

How can a vitamin fool a test?

Some lab tests use biotin to help catch and measure a substance in your blood. Extra biotin from a pill can get in the way. Think of it as extra pieces mixed into a puzzle: the test may give the wrong picture. [3]

Some thyroid test methods may show a falsely low TSH, the signal that tells the thyroid to work, or falsely high thyroid hormone levels. That pattern can look like an overactive thyroid on paper. The effect depends on the test method. [2]

The FDA has also warned about falsely low results on certain tests for troponin. This is a protein doctors check when they suspect heart damage. Some test makers have fixed this issue; others have not. It does not affect every blood test. [4]

Before your next blood draw

Take a photo of your supplement labels. Show your doctor and the lab the dose you take and when you last took it. Ask:

“Does this contain biotin? Could it affect my tests? Should I pause it, and for how long?”

There is no single wait time that fits everyone. It depends on the dose and the test. Kidney disease can also make biotin take longer to leave your blood. Let your care team set the timing. [3]

Do not stop a prescribed treatment on your own. And never delay urgent care to wait for biotin to clear. Tell the team what you took.

A surprising result still deserves a proper check. The useful step is to give your care team the whole picture, including the pill you bought for your hair.

For the bigger picture, Friday’s video walks through my supplement list: what to stop, keep, and watch, and why your kidney health changes the answer.

Related video: A Kidney Doctor’s Supplement List

With gratitude,

Dr. Sean

Join our free public AMA this Thursday, September 10, at 6 PM Pacific. Bring your questions about kidney health, food, and supplements. Everyone is welcome. No membership is needed. Visit the YouTube channel for the live Q&A.

The research behind this letter

1. NIH Office of Dietary Supplements. Biotin: Fact Sheet for Consumers.

2. Li D, et al. Biotin use and blood test results in healthy adults. JAMA. 2017;318:1150–1160. Original study.

3. Li D, et al. AACC guidance on biotin interference in lab tests. Journal of Applied Laboratory Medicine. 2020.

4. FDA. Biotin interference with troponin lab tests.

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This content is for education only and is not personal medical advice. Ask your healthcare provider about your own care. The views expressed are Dr. Hashmi’s own and do not represent his employer.

Never start, stop, or change the dose of a prescription medicine without your doctor’s guidance.