You generally don’t need to fast before a thyroid test, but a healthcare professional may advise you to prepare in other ways.

The thyroid gland sits at the front of your neck and helps control your metabolism. If this gland produces too much, or too little, thyroid hormone, it could indicate a thyroid disorder.

While certain imaging tests are available to diagnose these disorders, healthcare professionals more commonly check thyroid function with a blood test.

It’s not necessary to fast, or to go without eating, before a thyroid function test unless a healthcare professional recommends it.

In this article, you will learn why you might need a thyroid test and what special preparations — such as fasting — you may need to do beforehand.

A thyroid test is a simple blood test done at your doctor’s office or in a laboratory.

Technically, you don’t need fasting or other special preparations before you have your thyroid test.

But recent research suggests that meal timing, how long it’s been since your last meal, and the time of day can affect TSH levels. Healthcare professionals may need to consider this in regard to testing.

If you need to have regular thyroid tests, scheduling them at consistent times of day may offer better results.

There are no official guidelines that require fasting for a thyroid test, but if you’re having other blood work done alongside thyroid blood tests, it may be part of your general blood test preparation.

Be sure to ask a healthcare professional for any special instructions before your thyroid testing.

Some blood tests require you to fast, or not eat, for a certain length of time before the test. This is so your results will be more accurate, and not altered by any recent food.

Certain components of your blood, such as blood sugar, do change between meals and periods of fasting. Blood tests that require fasting include cholesterol or lipid tests, and blood glucose tests.

Particular types of foods, or any meal in general, can affect the results of these tests.

Other types of medical tests may also require fasting, but a healthcare professional will give you specific instructions when this is necessary.

Thyroid testing is not always part of a comprehensive health exam, but you can request it if you think you need it.

Usually, healthcare professionals order thyroid tests if you have a family history of thyroid issues, or you have symptoms that are common to thyroid disorders.

The two most common thyroid disorders are an underactive thyroid gland (hypothyroidism) and an overactive thyroid gland (hyperthyroidism).

You might want to request a thyroid test if you’re experiencing:

  • fatigue (low energy)
  • unexplained weight changes
  • joint or muscle pain and weakness
  • temperature intolerances (either hot or cold)
  • thinning hair
  • depression
  • nervousness or irritability
  • an enlarged area of your neck called a goiter

Symptoms of thyroid disease can also overlap with other disorders, but a thyroid test can confirm or rule out your thyroid gland as a source of these problems.

Thyroid tests measure how well your thyroid gland is working.

Blood tests measure the individual hormones produced by your thyroid gland:

  • Thyroid-stimulating hormone (TSH): This is usually the first test a doctor may order. The standard range for TSH is 0.5–5 mU/L , but this can vary based on age, gender, and stage of life.
  • T3: Healthcare professionals most often use a T3 test to diagnose hyperthyroidism. It measures the amount of the hormone triiodothyronine. They may order this test ordered if T4 levels are higher than usual.
  • T4: A T4 test checks the level of thyroxine in your body. Too much or too little can indicate thyroid disease.

Doctors usually only order imaging tests for thyroid issues if you have any irregularities or are experiencing symptoms.

Thyroid testing uses a sample of blood to measure levels of hormones made by the thyroid gland. These hormones help regulate growth and metabolism, and levels that are too high or too low could be a sign of thyroid disorder.

You don’t need to fast for thyroid function tests, although a healthcare professional may recommend it for you specifically, or if you need other blood work done at the same time.