Key takeaways:
Thyroid function tests are blood tests used to measure the health of your thyroid. Your thyroid is a small gland in your neck that plays a key role in metabolism.
TSH (thyroid-stimulating hormone) is one of the easiest ways to screen for thyroid problems. A high TSH may mean an underactive thyroid. A low TSH may mean an overactive thyroid.
There are several different tests that can measure thyroid function. They’re often measured together to get a full picture of your thyroid health.
Abnormal thyroid function test results don’t always mean you have a thyroid problem. Temporary fluctuations can happen with things like stress, illness, or even time of day.
Your thyroid plays an important role in a number of different processes in your body, including energy and metabolism, immune function, bone strength, and heart health, to name a few. So if something feels off or you’re experiencing new symptoms, your healthcare team may recommend a screen for thyroid problems.
Thyroid function tests are a quick and easy way to measure the health of your thyroid gland. They can help diagnose a number of different thyroid conditions — like an overactive or underactive thyroid. And they can also help figure out why your thyroid may be producing more or less hormone than normal. So we created this guide to help you understand what your results mean.
What do thyroid function tests measure?
Thyroid function tests check levels of hormones and antibodies in the blood. Most of the time, a healthcare professional will order several of these tests together to get the overall picture of your thyroid function.
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Below we describe the different types of thyroid tests, what they measure, and the role they play in your thyroid health.
Thyroid-stimulating hormone (TSH)
A thyroid-stimulating hormone (TSH) test is usually the best test to screen for problems with the thyroid:
A high TSH may mean the thyroid isn’t producing enough thyroid hormone. This condition is known as hypothyroidism.
A low level of TSH may mean the thyroid is producing too much thyroid hormone. This condition is known as hyperthyroidism.
TSH is a hormone that’s produced by your pituitary gland, which is located in your brain. It stimulates the thyroid gland to produce its two main hormones: thyroxine (T4) and triiodothyronine (T3).
The pituitary gland and thyroid gland work together in a feedback loop. When the pituitary gland senses low levels of thyroid hormone, it increases TSH production to stimulate the thyroid gland. If the pituitary senses high levels of thyroid hormone, the pituitary decreases its TSH production.
Thyroxine (T4)
T4 is one of the hormones that’s produced by your thyroid. It helps to regulate things like your energy level, mood, and body temperature. Sometimes, T4 measurements are divided into two categories:
Total T4: Total T4 is the entire amount of T4 hormone that exists in the body.
Free T4: This is the amount of hormone that isn’t bound to another protein, which means it’s in its active form. Free T4 is what actually affects the body’s tissues.
Triiodothyronine (T3)
T3 is another hormone that’s produced by your thyroid. T3 affects your metabolism, bone, and digestive health. Similar to T4, you may see total T3 or free T3 ordered as part of your labs:
Total T3: This is the entire amount of T3 hormone in the body.
Free T3: Free T3 is the level of hormone in your body that isn’t bound to another protein. Free T3 is what actually affects the body’s tissues.
T3 levels are more helpful for diagnosing an overactive thyroid (hyperthyroidism) than an underactive thyroid (hypothyroidism). This is because in hypothyroidism, T3 levels can sometimes still appear normal. And, since the level of free T3 in the body can be very small, measuring it can be unreliable and less helpful.
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Thyroid antibodies
Sometimes, the immune system may produce antibodies that attack the thyroid in certain autoimmune conditions. These include thyroglobulin antibodies (TgAb) and thyroid peroxidase antibodies (TPOAb). These tests are sometimes helpful in identifying the underlying cause of some thyroid conditions.
How to read thyroid lab results
Normal ranges for thyroid function tests may vary from lab to lab. Your results will usually provide a reference range that’s considered normal for each value. But in general, normal ranges are as follows.
| Value | Normal range |
|---|---|
| TSH | 0.4-4.5 mIU/mL |
| T4 |
|
| T3 |
|
| TPO Ab | Less than 34 IU/mL |
| Tg Ab | Less than 1.0 IU/mL |
Some institutions may use narrower ranges depending on how the blood is analyzed in the lab. Lab results usually list a reference range. This can help you know if your values are normal for that specific lab.
If your results are outside the normal range, it doesn’t necessarily mean you have a thyroid problem. This can happen with temporary fluctuations in thyroid function rather than a longer-term thyroid condition. And day-to-day variation can be normal and part of a healthy, responsive endocrine system.
What can affect thyroid test results?
Many factors may affect thyroid test results. Variation or abnormalities in thyroid test results could be related to:
Age: TSH levels tend to naturally increase with age.
Time of day: TSH levels are often higher in the evening.
Stress or recent illness: Both of these can lead to a temporary decrease in all thyroid function tests.
Fasting: Fasting can lead to higher TSH values.
Pregnancy: Thyroid levels change throughout pregnancy. Different reference levels are recommended for each trimester to account for these changes.
Medications: Several medications may interfere with thyroid test results, including heparin, phenytoin, and amiodarone. And any medication that contains estrogen — like birth control pills — can falsely elevate total T3 and T4 levels.
Biotin supplements: Some people take this for skin and hair health, and it’s known to falsely decrease your TSH level. It’s also found in most multivitamins, but the amount is usually too small to make an impact.
Be sure to discuss any abnormal results with your healthcare team. They can help you figure out what your results mean, or if they should be repeated on a different day.
Frequently asked questions
Your thyroid function tests may or may not include T3 levels. This is because they aren’t always helpful in diagnosing thyroid conditions. The level of free T3 in the body can be very small, so measuring it can be unreliable. And T3 can also sometimes appear normal, even when the thyroid isn’t functioning properly.
A high TSH can mean your thyroid gland isn’t producing enough thyroid hormone, a condition called hypothyroidism. Symptoms of low thyroid function include:
Fatigue
Weight gain
Hair loss
Feeling cold all the time
Difficulty thinking or concentrating
Irregular periods
You don’t need to avoid specific foods if you have a high TSH. But a balanced, nutritious diet can help support your thyroid health. So if you can, avoid foods that are linked to inflammation and chronic disease. These include foods that are high in:
Sugar
Salt
Saturated fat
Your thyroid function tests may or may not include T3 levels. This is because they aren’t always helpful in diagnosing thyroid conditions. The level of free T3 in the body can be very small, so measuring it can be unreliable. And T3 can also sometimes appear normal, even when the thyroid isn’t functioning properly.
A high TSH can mean your thyroid gland isn’t producing enough thyroid hormone, a condition called hypothyroidism. Symptoms of low thyroid function include:
Fatigue
Weight gain
Hair loss
Feeling cold all the time
Difficulty thinking or concentrating
Irregular periods
You don’t need to avoid specific foods if you have a high TSH. But a balanced, nutritious diet can help support your thyroid health. So if you can, avoid foods that are linked to inflammation and chronic disease. These include foods that are high in:
Sugar
Salt
Saturated fat
The bottom line
The thyroid is an important gland in your body that regulates your energy, metabolism, and growth. Thyroid function tests can help measure how well the thyroid gland is working. These tests can be helpful both for people with known thyroid conditions as well as those who are experiencing new or unexplained symptoms. But interpreting these results can be complicated, since so many different factors can affect them from day to day. Your healthcare team can help you figure out the meaning behind any abnormal results.
Why trust our experts?
References
American Thyroid Association. (n.d.). Thyroid function tests.
Basile, L. M., et al. (2020). What are T3, T4, and TSH? EndocrineWeb.
Haugen, B. R. (2009). Drugs that suppress TSH or cause central hypothyroidism. Best Practice & Research: Clinical Endocrinology & Metabolism.
LabCorp. (2021). Thyroglobulin, Comprehensive (with Anti-Tg Screen and Reflex to RIA) (Endocrine Sciences).
LabCorp. (2021). Thyroid peroxidase (TPO) antibodies.
Novkovic, B. (2021). TSH blood test: Normal levels & range. SelfDecode.
Soh, S. B., et al. (2019). Laboratory testing in thyroid conditions - pitfalls and clinical utility. Annals of Laboratory Medicine.
Samuels, M. H. (2001). The use of tests for the diagnosis and monitoring of thyroid disease. Laboratory Medicine.
Welsh, K. J., et al. (2016). How reliable are free thyroid and total T3 hormone assays? European Journal of Endocrinology.
