How to Prepare for Thyroid Tests

Do you need to fast before thyroid tests?

According to MedlinePlus, fasting isn't required for TSH, FT4 or FT3 tests on their own. If your doctor ordered other blood tests at the same time, you may need to avoid food and drink for a few hours beforehand, because those other tests require it.

What time of day is best

The levels of many hormones in the blood fluctuate over the course of the day, so it's advisable to have this kind of test done in the morning, before around 10 a.m., according to Medicina Practica's laboratory guidance. A specific laboratory may give you different instructions, so if your referral or lab specifies another time, follow that instead.

Step by step: from a few days before to right after the draw

The Lithuanian laboratory Antėja recommends the following preparation, based on how far in advance each step applies:

• A few days before: if you take supplements containing biotin (vitamin B7/H), it's recommended to stop them at least 48 hours before the test, since biotin can distort the results of free T4 and other thyroid tests.
• A few days before: tell your doctor about all the medications and supplements you're taking. Don't stop a doctor-prescribed medication on your own — but it's worth discussing whether anything needs adjusting before this particular test.
• The day before: avoid alcohol and intense physical exertion, as these can affect the concentration of some blood markers.
• Morning of the test: it's best not to take your L-thyroxine dose on the same day before the blood draw, unless your doctor has told you otherwise — you can take it once you're back home.
• Morning of the test: if allowed, you can drink plain still water before the sample is taken; it's better to avoid coffee, tea or juice if other blood tests requiring fasting are being drawn at the same time.
• Right before the draw: sit calmly for a few minutes beforehand and try to avoid stress — nervousness and physical exertion can temporarily affect the levels of some hormones in the blood.
• After the draw: press on the puncture site and hold for about 5 minutes or longer; it's recommended to leave the plaster on for at least an hour before removing it.
• After the draw: avoid heavy lifting and intense exercise afterward, so a bruise doesn't form at the puncture site.

Medications and supplements to mention

Before the test, it's worth telling the lab or your doctor about any medications and supplements you're taking, especially biotin (vitamin B7/H) and L-thyroxine, since both can affect the result. Don't stop a doctor-prescribed medication on your own — discuss any change with your doctor first.

What can go wrong if you don't prepare

If you don't stop biotin (vitamin B7/H) supplements before the test, biotin can interfere with several lab tests, including free thyroxine (FT4), and skew the result — which can in turn mislead your doctor's assessment. That's why it matters to mention biotin use in advance and stop it early enough.