Quick answer

Ask your pharmacist or doctor before combining. Of the eight B vitamins a B complex can contain, the documented issue is biotin. The levothyroxine label says biotin interferes with the thyroid blood tests used to check your dose, and tells patients to stop biotin-containing supplements for a time before testing.^1,2

Key facts

NIH lists B-complex supplements among the products that contain biotin.²
The levothyroxine label tells patients to stop biotin and any biotin-containing supplement before thyroid function tests.¹
FDA says biotin intakes above the recommended daily amount can make some lab tests read falsely high or falsely low.³
The label's warning names biotin only; the sources on this page do not describe the other B vitamins altering thyroid tests.¹

Ask your pharmacist or doctor first

B complex supplements can contain biotin. The levothyroxine label says biotin interferes with thyroid hormone tests built on biotin and streptavidin, which may give wrong results, and tells patients to stop biotin and any biotin-containing supplement for a time before thyroid function testing.^1,2

On the medicine's label

Which ingredient is the concern

A B complex can contain up to eight B vitamins. The one the levothyroxine label names is biotin, also called vitamin B7. Many lab tests, including thyroid hormone tests, use biotin as part of how they work, and extra biotin in a blood sample can throw the reading off.^1,3

Why it matters for your dose

Levothyroxine doses are checked and adjusted with thyroid blood tests, mainly TSH and T4. A result distorted by biotin could point toward a dose change you do not need, or hide one you do.^1,4

The label therefore tells prescribers to ask about biotin, and it lists biotin under interactions between the medicine and lab tests rather than under interactions with other medicines.¹

How to check your B complex

Look for biotin on the Supplement Facts panel; it may also be called vitamin B7³
Note the amount and the percent Daily Value; the Daily Value for biotin is 30 mcg²
Check your other products too, such as a multivitamin, a prenatal vitamin or a hair, skin and nail product³
If biotin is not listed, still tell your pharmacist what the product does contain

What about the other B vitamins?

The sources on this page do not describe thiamin, riboflavin, niacin, pantothenic acid, vitamin B6, folate or vitamin B12 changing levothyroxine or thyroid test results. That is not a finding either way, so list every vitamin you take when you speak to your pharmacist, as MedlinePlus advises.^1,4

Questions for your pharmacist

Does my B complex, or anything else I take, contain biotin?
Should I pause it before my next thyroid test, and for how long?
Do I need to tell the lab as well as you?
Is there anything else in my supplements that affects levothyroxine?

Questions people ask

Is it only high-dose biotin that matters?

The label's instruction covers biotin and biotin-containing supplements without setting a minimum amount, and FDA says intakes above the recommended amount can affect some tests. Ask your doctor how it applies to your product.^1,3

Can I take B complex at the same time of day as levothyroxine?

That is a timing question for your pharmacist, who can look at your whole schedule and every product you take. The sources on this page set no timing rule for B vitamins and levothyroxine.⁴

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.