Most thyroid tests don’t require any special preparation. However, the radioactive iodine uptake scan requires you to abstain from certain foods and medications for up to several weeks.

If a doctor has scheduled a thyroid test, you may need to take specific steps before the test. Always follow your doctor’s instructions, but these general guidelines can be helpful as you prepare.

No special preparation is needed for thyroid function tests. These are typically a single blood draw.

Although not necessary, you can take steps to make the blood draw easier. For example, staying hydrated can make it easier for the person drawing your blood to access a vein. Avoid energy drinks, alcohol, and coffee. Opt for water as often as possible.

Research also suggests that taking biotin (aka vitamin B7 or vitamin H) supplements may affect the results of a thyroid function test. You may need to avoid taking these supplements for a few days before the test.

Doctors often order other tests together with thyroid function tests. You may need to follow specific instructions to prepare for those tests.

What to tell your doctor before your thyroid function test

Before you take a thyroid function test, be sure to let your doctor know:

  • all the medications or supplements you’re currently taking
  • if you’re ill or have had any recent illnesses
  • are pregnant or might be pregnant
  • have had an X-ray or other test containing iodine recently

You’ll most likely only have a thyroid antibody test if you’ve already had a thyroid function test.

No special preparation is necessary for a thyroid antibody test. Like thyroid function tests, this test is a single blood draw.

Depending on the purpose of an RAIU and scan, your doctor may ask you to consume a low iodine diet for 1 to 2 weeks. This may involve avoiding iodine-rich foods.

Examples of common foods that may contain iodine include:

  • seafood
  • iodized salt and salty snacks, like chips or pretzels
  • dairy, soy, and eggs
  • white bread

You’ll also need to avoid taking certain vitamins, supplements, or medications that contain iodine.

If you take antithyroid medications, like methimazole or propylthiouracil, you’ll need to stop these 5 days before the test.

The exact timing for when to stop taking medications can vary, so check with your doctor.

You’ll also need to fast for at least 4 hours before the first part of the procedure (the uptake). You’ll need to fast for 1 hour after the uptake as well. The scan takes place 24 hours after the uptake.

The turnaround time for thyroid test results can vary. Thyroid function test results are usually available more quickly than other thyroid tests, but it can depend on the lab.

Your doctor will review your test results with you and discuss what they mean and what the best next steps might be.