What are Thyroid Hormone Blood Tests?

The thyroid is a butterfly-shaped gland located near your Adam's apple that produces the hormones triiodothyronine (T3) and thyroxine (T4).4 T3 and T4 regulate all aspects of your metabolism, such as the rate at which your body uses carbohydrates, your body temperature, and your protein production.3

When your thyroid does not produce enough of these hormones (hypothyroidism), you may experience a lack of energy, weight gain, and depression. 2 Thyroid function blood tests--also known as thyroid hormone blood tests--can determine whether your thyroid is functioning correctly.7 A healthcare provider can administer these tests to help determine what's causing your symptoms and which treatment may work best for you.

Types of Thyroid Function Tests

The set of blood tests that evaluates your thyroid function is known as a full thyroid panel.1 A full thyroid panel may include these tests: 2,6,7,8

  • Thyroid stimulating hormone (TSH): Detects hypothyroidism. If your hormone levels are low, your TSH levels will be high.
  • Thyroxine (T4): Low levels of this hormone indicate hypothyroidism.
  • Microsomal thyroid antibodies (TPO): If these antibodies are present, you may have hypothyroidism.
  • Reverse T3 (RT3): This inactive mirrored form of T3 inhibits T3 function. Maintaining the healthy ratio of T3 to RT3 is important - overproduction of RT3 can cause severe hypothyroidism.6

If concerned that you may have thyroid issues, your physician will likely order a T4 or TSH test. If you are experiencing hypothyroid symptoms while your TSH and T4 levels appear normal, it is important that you get your RT3 levels tested as well.6

If the results of those tests are abnormal, further tests may pinpoint the problem.2 For example, the conversion of T4 into its active T3 form may be affected by gut health, and a high amount of RT3 may form due to these conversion issues.

Thyroid Blood Test Procedure

Regardless of the type of thyroid function blood test(s) you undergo, they all involve taking a blood draw (venipuncture) in a doctor's office. Studies have shown that patients who partake in early morning fasting have higher TSH levels than afternoon patients who do not fast.5

Certain medication can affect your thyroid blood test results, including:5

  • Those that cause hypothyroidism, such as iodine supplements, lithium, tyrosine kinase inhibitors, and immunomodulating drugs like interferon alpha
  • Those that interfere with thyroid hormone replacement absorption in the gut, like calcium carbonate, iron sulfate, and proton pump inhibitors
  • Those that interfere with lab measurements, but not the thyroid itself, such as certain nonsteroidal anti-inflammatories (NSAIDs), heparin, and biotin

Pregnancy and severe illness can also affect thyroid function.5 Before undergoing a thyroid blood test, tell your healthcare provider if you're taking medication, are pregnant, or have been hospitalized recently.

Request Your Appointment Today

If you're feeling tired or find yourself gaining weight, thyroid issues may be to blame.2 Thankfully, a thyroid hormone blood test may help you discover the cause of your symptoms.7 To find a thyroid doctor near you who performs thyroid function tests, click here.

Frequently Asked Questions

What blood test shows thyroid function?

Many different blood tests can determine the health of thyroid function, such as those for thyroid stimulating hormone (TSH), thyroxine (T4), thyroid antibodies, and reverse T3 (RT3).2,6,7,8 For the best diagnosis you may undergo several of these tests, known as a full thyroid panel. 1

What is included in a thyroid panel?

A thyroid panel includes multiple blood tests that are designed to evaluate your thyroid function and diagnose various thyroid-related conditions. A thyroid panel might include a thyroid-stimulating hormone (THS) test, a thyroxine (T4) test, and a thyroid antibody test. 1

Do I have to fast for a thyroid blood test?

Studies have shown that early morning fasting causes higher TSH levels compared to tests taken in the afternoon on patients that did not fast. Since fasting affect TSH levels, fasting might make it difficult to diagnose subclinical hypothyroidism, as the condition is diagnosed by specifically looking at TSH levels.5 Ask your healthcare provider whether you should fast before taking your thyroid blood test.


1. ADAM. "Everything That You Need To Know Before Getting A Full Thyroid Panel." ThyroMate. ThyroMate.com, April 25, 2017, https://www.thyromate.com/blog/everything-that-you-need-to-know-before-getting-a-full-thyroid-panel. Accessed April 3, 2018.

2. Luo, Elaine K. "Thyroid Function Tests." healthline. Healthline Media, February 27, 2018, https://www.healthline.com/health/thyroid-function-tests. Accessed April 3, 2018.

3. Mayo Clinic Staff. "Hypothyroidism (underactive thyroid)." MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/hypothyroidism/symptoms-causes/syc-20350284. Published December 6, 2017. Accessed April 3, 2018.

4. Northrup, Christiane. "What is Thyroid Disease? Common Thyroid Disease Symptoms To Look For." Christiane Northrup, M.D. Christiane Northrup, October 24, 2017, https://www.drnorthrup.com/thyroid-disease/. Accessed April 3, 2018.

5. Shomon, Mary. "Factors That May Affect Your Thyroid Test Results. Fasting, Medication, Pregnancy, and Severe Illness May Be the Culprits." verywellhealth. Richard N. Fogoros. March 3, 2018, https://www.verywellhealth.com/optimum-time-and-conditions-for-thyroid-blood-tests-3232911. Accessed April 3, 2018.

6. The National Academy of Hypothyroidism. "Thyroid Health - Understanding Reverse T3." nahis. National Academy of Hypothyroidism, 20 April 2017, https://www.nahypothyroidism.org/thyroid-health-understanding-reverse-t3/. Accessed 17 April 2018.

7. "Thyroid Blood Tests." Cleveland Clinic. The Cleveland Clinic Foundation, https://my.clevelandclinic.org/health/diagnostics/17556-thyroid-blood-tests. Accessed April 3, 2018.

8. "Thyroid Function Tests." American Thyroid Association. https://www.thyroid.org/thyroid-function-tests/. Accessed April 3, 2018.