Thyroid Function Tests: Types, Purpose, and Useful Considerations

Thyroid Function Tests: Types, Purpose, and Useful Considerations

Phlebotomy procedure during coronavirus crisis. medical worker preparing to take blood for analysis
Dr. Natalie Bessom
Medically reviewed by

Natalie Bessom, D.O. Board-certified family medicine doctor with specialty training in nutrition, USA

The thyroid gland, a butterfly-shaped organ at the base of your neck, plays a vital role in regulating your body’s metabolism, energy levels, mood, and even heart rate. When it’s not functioning properly, it can cause a cascade of symptoms that can significantly impact your quality of life.

This is where thyroid tests come in. Thyroid function tests (TFTs) are a series of blood tests used to measure how well the thyroid gland is working. These tests are necessary for detecting both thyroid problems such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid), as well as other thyroid-related conditions such as thyroid cancer or nodules.

In this article, we will answer the following questions:

What are thyroid function tests looking for?

TFTs are used to assess the biochemical function of the thyroid gland. This means that they measure blood levels of different hormones and proteins related to your thyroid function.

Of course, we cannot talk about thyroid function tests without mentioning the two main players involved in thyroid function. These are:

  • Thyroid Hormones: Triiodothyronine (T3) and thyroxine (T4) are hormones produced by your thyroid gland. They travel through your bloodstream and influence various bodily functions.
  • Thyroid-Stimulating Hormone (TSH): Produced by the pituitary gland at the base of your brain, TSH acts like an instructor, telling your thyroid how much T3 and T4 to produce.

One main defining feature of TFTs is that they are purely blood tests focused on hormone levels and function, distinguishing them from other tests like thyroid imaging tests which are used to visualize the thyroid’s size, shape, and position, detect nodules, or assess the gland’s ability to take up iodine.

In the next section, we will go into detail about what a full thyroid panel entails.

What are the types of thyroid tests and how are they relevant?

TSH Test 

This is often the first test done because it’s the most sensitive indicator of thyroid function. Usually, high TSH level indicates hypothyroidism (when FT4 is low), while a low TSH level suggests hyperthyroidism (when FT4 is high). Normal TSH with abnormal T4 or T3, however, may suggest subclinical thyroid disease or a problem with the pituitary gland.


Free T4 (FT4) Test

This measures the amount of circulating T4 hormone not bound to proteins, thus available to the body’s tissues.  It’s a more precise measure of thyroid function compared to total T4 levels. FT4 levels can be abnormal even with a normal TSH in some cases. Usually, a  high FT4 level usually accompanies hyperthyroidism, and a low FT4 level can be a sign of hypothyroidism.


Free T3 (FT3) Test

This measures the level of T3 that is free and available for the body’s use. It’s not always included in the initial testing but might be helpful in specific situations. For instance,  FT3 testing might be added if the initial results are ambiguous or if a condition like hyperthyroidism is suspected, where T3 levels can be disproportionately elevated


Total T4  (TT4) and Total T3 (TT3) Tests

These tests measure the total levels of T4 and T3 in the blood, including both the free and the protein-bound hormone. These tests are less commonly performed and are usually ordered to provide a more comprehensive view of thyroid hormone function, especially in situations where binding protein abnormalities are suspected.


Thyroid Antibody Tests

These tests detect the presence of antibodies that attack the thyroid gland. They are particularly useful for diagnosing autoimmune thyroid diseases like Hashimoto’s thyroiditis or Graves’ disease. They aren’t routinely included in initial screenings unless symptoms suggest an autoimmune condition. Here are the antibodies of interest:

  1. Thyroid Peroxidase Antibodies (TPOAb): These antibodies target an enzyme in the thyroid gland involved in hormone production. High levels are commonly associated with Hashimoto’s thyroiditis.
  2. Thyroglobulin Antibodies (TgAb): These target thyroglobulin, a protein involved in thyroid hormone production. Elevated levels can suggest Hashimoto’s thyroiditis and are also monitored in thyroid cancer management. Thyroglobulin antibodies  can interfere with thyroglobulin (Tg) measurements used as a tumor marker for thyroid cancer recurrence, requiring careful monitoring and interpretation in patients post-treatment.
  3. Thyroid Stimulating Hormone Receptor Antibodies (TSHRAb or TRAb): These include stimulating and blocking antibodies. Stimulating antibodies can lead to Graves’ disease by mimicking TSH and causing excess thyroid hormone production, while blocking antibodies can inhibit thyroid function.
Reverse T3 (rT3) Test

This test measures a form of T4 that is inactive. It’s not typically part of standard thyroid function panels but can be important in the evaluation of thyroid function in certain conditions, such as severe illness or starvation, where the body converts T4 to rT3 instead of the active T3. It helps assess how the body is managing and converting thyroid hormones, especially in complex cases with non-thyroidal illness syndrome (NTIS).

What are reference ranges?

Reference ranges are absolutely important in interpreting thyroid function test results as they provide the benchmarks that help determine whether thyroid hormone levels are normal, high, or low. These ranges, however, are not one-size-fits-all; they can vary based on the testing methods used by different laboratories, patient age, sex, and other physiological factors such as pregnancy.

Here are some key facts about reference ranges:

  • Different labs may use different test methods or equipment, which can influence the reference ranges.
  • Reference ranges are often established based on a healthy population that is assumed to be representative of the general public. However, what is normal in one demographic or ethnic group may not be for another.
  • Factors like age and sex can affect hormone levels. For example, older adults typically have higher TSH levels than younger people.

Understanding these nuances is crucial because relying solely on standard reference ranges without considering individual and methodological variations can lead to misinterpretation of thyroid function. This is why healthcare providers often look at trends in individual test results over time and correlate them with symptoms and other test findings rather than making decisions based solely on single test results.

Physicians may also adjust the “normal” range for individuals based on clinical judgment and the specific health context. For instance, a slightly elevated TSH may be acceptable in an elderly person without symptoms, but the same result might prompt treatment in a younger person with symptoms of hypothyroidism.

When is a thyroid panel necessary?

Your doctor might recommend thyroid testing if you exhibit symptoms of thyroid dysfunction, have a family history of thyroid disease, or have a medical condition that affects the thyroid. 

Here’s a summary of some recommendations for thyroid function screening from various medical organizations:

Also, if  you have already been diagnosed with a thyroid disorder and are on medication, regular thyroid testing helps ensure that the medication is working effectively.

What to expect during a thyroid test

A thyroid test is a simple procedure. A healthcare professional will draw a small amount of blood from your arm. There’s usually no special preparation required, although your doctor might advise you to temporarily limit iodine-rich foods and avoid medications containing iodine for about a week beforehand in order to ensure accurate test results. After your blood is drawn, it is then sent to a laboratory for analysis and results.


The doctor's hand in the laboratory holds a test tube with blood

What to do after your test

Your doctor will discuss your results with you. They will consider your symptoms, medical history, and the complete picture of your test results to make a diagnosis and recommend the next steps. This might involve additional tests, medication, or lifestyle changes. Do not hesitate to voice out any concerns you might have.

Key Takeaways

  • TFTs, also collectively referred to as a thyroid panel, are blood tests used to assess your thyroid function and diagnose any underlying conditions.
  • While also useful in thyroid evaluation, thyroid imaging tests (which include procedures like ultrasound, radioactive iodine uptake tests, and thyroid scans) are not considered part of the thyroid function tests.
  • A full  thyroid panel typically includes various tests, including:
    • TSH test
    • Free T4 test
    • Free T3 test
    • Total T3 test
    • Total T4 test
    • TPOAb test
    • TgAbtest
    • TSHRAb test
  • While not part of a standard thyroid panel, the rT3 test is especially useful in the diagnosis of non-thyroidal illness syndrome.
  • Reference ranges for thyroid tests can vary by lab and individual factors, which is why doctors consider these differences when reading your results.

If you want to better manage your blood tests, medications, symptoms, moods, energy levels, and weight changes, download our ThyForLife app to get a head start! ThyForLife is an all-in-one thyroid solution right at your fingertips.


At ThyForLife, we do our utmost to provide accurate information. For detailed medical information regarding diagnosis, treatment, and general practices please consult your healthcare professional. Always listen to the advice of your healthcare provider.
Share this article

You might also like

Showing most popular and related articles for you

Be the first to get

weekly thyroid related articles and useful tips directly to your inbox!