What Your Labs Aren’t Telling You

How supplements and medications can quietly change your test results.

You fast the night before. You show up on time. You roll up your sleeve and do everything right. So when a lab result comes back abnormal, it’s natural to assume it means something is wrong.

But sometimes, the result isn’t wrong because of your health. It’s wrong because of something you took that morning, or even days earlier — a vitamin, a supplement, an over-the-counter pill — that quietly changed the number on the page.

This happens more often than most people realize. And it can go both ways: it can make a healthy result look alarming, or it can make a real problem look normal. Either way, it can send you and your doctor down the wrong path.

Here’s what you need to know before your next test.

The supplement almost no one thinks about: biotin

Biotin, also called vitamin B7, is one of the most common ingredients in hair, skin, and nail supplements — and it is also one of the most well-documented causes of inaccurate lab results.

The FDA has issued more than one public warning about this. Biotin can interfere with a type of lab test called an immunoassay, which is used to measure many common markers, including thyroid hormones and troponin, the marker used to help diagnose a heart attack.

The FDA has received reports of biotin causing falsely low troponin results — including at least one report where this may have contributed to a missed diagnosis. Biotin has also been linked to inaccurate results for thyroid tests, vitamin D, vitamin B12, ferritin, testosterone, and several other common markers.

The supplements involved aren’t exotic. Many hair, skin, and nail products contain biotin doses far above what your body actually needs — sometimes hundreds of times higher. If you take one of these, or a multivitamin that includes biotin, your lab results could be affected without you ever knowing why.

What to do

Tell your provider and the lab if you take biotin in any form, including multivitamins and hair, skin, or nail supplements. Many providers recommend stopping biotin for two to three days before bloodwork, though you should always confirm this with your provider rather than stopping supplements on your own.

Other common supplements worth mentioning

Biotin gets the most attention, but it isn’t the only one. Here are other supplements known to affect common tests:

• Vitamin C (high dose). Can throw off some glucose readings and certain stool tests used to screen for colorectal cancer or digestive bleeding.
• Calcium supplements. Taking calcium before a bone density scan can make your bones appear denser than they really are.
• Creatine. Commonly used for muscle and exercise support, creatine can raise a marker called creatinine, which is used to check kidney function — potentially making healthy kidneys look impaired.
• Iron supplements. Can cause a false-positive result on a stool test used to check for hidden digestive bleeding.
• Fish oil and niacin. Both can genuinely change cholesterol and triglyceride numbers, which is useful to know if you’re trying to track your true baseline.

This is not a complete list, and new research comes out regularly. The point isn’t to memorize every interaction — it’s to understand that supplements, even ones that seem harmless, can influence what your test results show.

It’s not just supplements

Prescription and over-the-counter medications can affect lab results too. A few well-documented examples: certain antibiotics can cause false readings on kidney and urine tests. Acid-reducing medications, a category that includes common heartburn drugs, have been linked to changes in certain hormone and breath-test results, and may affect blood-thinner monitoring in some patients. Even some blood thinners themselves can affect clotting-related lab values in ways your provider needs to account for.

The theme is the same as with supplements: what you take can change what the test shows, and your provider can only account for it if they know about it.

What this means for you

None of this means you should stop taking your supplements or medications on your own, or that you should distrust every lab result you’ve ever received. Most of the time, your results reflect exactly what’s happening in your body.

What it does mean is this: a full and honest list of everything you take is one of the most valuable pieces of information you can hand your provider — every single time you’re tested.

Before your next blood draw, test, or scan, consider bringing a simple list that includes:

• Every prescription medication you take
• Every over-the-counter medication, even ones you only take occasionally
• Every vitamin and supplement, including multivitamins
• Any recent change — something new you started, or something you recently stopped

If a result ever comes back and doesn’t match how you feel, or doesn’t make sense given your history, it’s fair to ask your provider directly: “Could anything I’m taking have affected this result?” That single question has, in documented cases, changed a diagnosis.

You know your whole story. Make sure your provider does too.

Reading your medical records and asking good questions only works if the information going in is complete. A lab result is only as accurate as the full picture behind it — and you’re the only one who knows everything you’re taking.

Keep a current list. Bring it every time. It’s a small habit that can prevent a very large misunderstanding.

---