Biotin and Thyroid Tests: Why Your Results Can Be Wrong

Yes, biotin can make thyroid test results wrong. High-dose biotin, the kind found in many hair, skin and nail supplements, can interfere with the way some thyroid tests are run, making TSH read falsely low and T4 or T3 read falsely high. Biotin does not change your thyroid. It changes the test. If you take it, tell your doctor and the people doing your test before you have thyroid tests done.

If you live with Hashimoto's, an underactive thyroid or no thyroid, and you have been taking a hair supplement because your hair is thinning, this matters more than most people realise. Here is what happens, why it can lead to the wrong decision, and what to do about it.

How biotin gets into the test

Many thyroid tests use biotin as part of the chemistry that measures your hormones. When there is a lot of extra biotin in your sample, it can throw that chemistry off, in a predictable direction:

• TSH can read falsely low.
• Free T4 and free T3 can read falsely high.
• TSH-receptor antibodies, the test used for Graves' disease, can also read falsely high.

Put together, that pattern looks like an overactive thyroid, even when the thyroid is fine. Australian pathologists reported in 2016 that biotin had led to people with healthy thyroids being started on medicine for an overactive thyroid they did not have.

Why it matters if you take levothyroxine

A low TSH with a high-looking T4 is also what a levothyroxine dose that is too high looks like. So for someone on thyroid medication, biotin interference could prompt a dose reduction that was never needed.

It can work the other way too. Because biotin pushes TSH down, it can hide a TSH that should be high, so a genuinely underactive thyroid can be missed.

After thyroid cancer, there is another test to think about. In a 2021 study that included people on levothyroxine after thyroidectomy, 10 mg of biotin a day lowered TSH by about a third and pushed thyroglobulin, the marker used to watch for thyroid cancer returning, down by more than half on the test systems that use biotin chemistry.

The Australian prescribing information for levothyroxine says the people doing your thyroid test should be told if you take biotin, because it can make results read falsely high or low.

How much biotin causes a problem?

Adults need about 25 to 30 micrograms of biotin a day, which a normal diet easily covers. Many hair, skin and nail products contain 5,000 to 10,000 micrograms. In Australia, biotin is sold on its own in strengths from 3,000 to 10,000 micrograms a tablet or capsule, and it is also in many "hair, skin and nails" blends.

That is the range that causes trouble. In a 2017 study, six healthy adults took 10 mg (10,000 micrograms) of biotin a day for a week. On one widely used test system their TSH dropped from a normal 1.6 to 0.1, low enough to look like an overactive thyroid. A week after stopping, their results were back to normal.

Does it affect every test?

No, and it is less of a problem than it was. The effect depends on the test system your sample is run on. Some systems do not use biotin chemistry at all, and since 2017 several manufacturers have changed their thyroid tests so they are much less affected. But not all have, and you will not know which system your sample goes through. That is why telling them matters.

How long should you stop before a thyroid test?

There is no single agreed answer, so it is worth asking rather than guessing:

• Australian pathology guidance ranges from about 3 days to a week. A 2025 Australian Prescriber article suggests at least a week.
• US guidance can be shorter for lower doses, and the US regulator has said there is not enough information to recommend a safe window at all.

Ask your doctor or the pathology collection centre what they want for your particular test, and stop only if they say so. If you take biotin for a medical condition, do not stop it without talking to your doctor.

Why nobody told you

You will not find a biotin warning on most Australian thyroid test information. healthdirect's thyroid test page simply asks you to tell your doctor about any medicines or supplements, and the main Australian pathology information pages for TSH and free T4 list no preparation. The levothyroxine prescribing information does mention it, but most people never read that far. So if this is news to you, you are not alone.

What to do

1. Check your supplements, including multivitamins and "hair, skin and nails" products, for biotin (sometimes listed as vitamin B7 or vitamin H).
2. Tell your doctor and the people doing your test that you take it, and how much.
3. Ask how long, if at all, they want you to stop before your next thyroid test.
4. If a result looks surprising, especially a low TSH, ask whether biotin could be the reason before any change to your dose.
5. Never change your levothyroxine dose yourself based on a result.

Does biotin help thinning hair?

There is little evidence that biotin helps hair unless you are low in it, and true deficiency is rare on a normal diet. If your hair is thinning with an underactive thyroid, our guide to hair loss on thyroid medication covers the things more worth checking.

Where ThyroBase fits

No nutrition product should change your thyroid test results. ThyroBase is a daily morning and evening nutrition powder for people living with Hashimoto's, an underactive thyroid or no thyroid, designed to sit alongside your thyroid medication, never instead of it. Here is who ThyroBase is, and is not, designed for.

Biotin does not change your thyroid. It can change your test.

Tell the people doing your test, before the result changes your dose.

Frequently asked questions

Can biotin affect thyroid test results?

Yes. High-dose biotin, common in hair, skin and nail supplements, can make TSH read falsely low and free T4 and T3 read falsely high on some test systems. The pattern can look like an overactive thyroid. Tell your doctor and the people doing your test if you take it.

How long should I stop biotin before a thyroid test?

There is no single agreed window. Australian pathology guidance ranges from about 3 days to a week, depending on the dose and the test. Ask your doctor or the collection centre what they want for your test, and stop only if they say so.

Can biotin make it look like my levothyroxine dose is too high?

It can. A falsely low TSH with a high-looking T4 is the same pattern as a dose that is too high, so biotin can prompt an unneeded dose reduction. If you take biotin, mention it before any change to your dose, and never adjust your dose yourself.

How much biotin is in hair supplements?

Many hair, skin and nail products contain 5,000 to 10,000 micrograms of biotin, compared with the 25 to 30 micrograms a day adults need. Studies showing test interference used 10,000 micrograms a day, a dose sold over the counter in Australia.

The honest takeaway

Biotin is not bad for your thyroid, but it can make your thyroid tests wrong, and on levothyroxine a wrong result can lead to a wrong dose. If you take a hair, skin and nail supplement, check it for biotin, tell your doctor and the people doing your test, and ask whether to pause it before your next test. Then let the result be about your thyroid, not your supplement.

ThyroBase is a nutritional supplement and is not intended to diagnose, treat, cure, or prevent any disease. It is designed to sit alongside your thyroid medication, not replace it. This article is general information, not medical advice. Never start, stop or change a supplement or prescription medication without speaking to your doctor or pharmacist first.

References

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Eutroxsig (levothyroxine sodium) Product Information and Consumer Medicine Information. Australian Commission on Safety and Quality in Health Care, Medicine Finder. 2025.
US Food and Drug Administration. Update: the FDA warns that biotin may interfere with lab tests. Safety communication. 2017, updated 2019.
National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand: biotin. eatforhealth.gov.au.
healthdirect Australia. Thyroid function tests. healthdirect.gov.au. 2025.