Thyroid Function Test: How to Interpret Your Results - GoodRx (2024)

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. The way to check if your thyroid is working properly is to measure thyroid function tests. These are a set of blood tests that show how well the thyroid is making and responding to thyroid hormone.

Some people have these tests performed when they have a known thyroid disorder, like hypothyroidism or hyperthyroidism. Other times, a provider might order these tests if you are experiencing symptoms without a clear explanation. If you have recently had thyroid function tests, we are here 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. And, most of the time, a provider will order several of these tests together to get the overall picture of your thyroid function.

Here are the different types of thyroid tests, what they measure, and the role they play in your thyroid health:

Thyroid-stimulating hormone (TSH)

TSH is a hormone that is 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, so each knows how much hormone to produce. When the pituitary gland senses low levels of thyroid hormone, it increases TSH production to stimulate the thyroid gland. On the other hand, if the thyroid is producing high levels of thyroid hormone, the pituitary decreases its TSH production.

This means that if you have a high level of TSH, it may mean that your thyroid is not producing enough T3 or T4. If you have a low level of TSH, it may mean that your thyroid is producing too much T3 or T4.

Thyroxine (T4)

T4 is one of the hormones that is 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 and free T4.

Total T4 is the entire amount of hormone that exists in the body. Free T4 is the amount of hormone in your body that isn’t bound to another protein, which means it is in its active form. Free T4 is what actually affects the body’s tissues.

Triiodothyronine (T3)

T3 is another hormone that is produced by your thyroid. T3 affects your metabolism and bone and digestive health. Similar to T4, you may see total T3 or free T3 ordered as part of your labs. Total T3 is the entire amount of hormones that exists in the body. 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 are usually the last test to become abnormal. And, even then, since the level of free T3 in the body can be very small, measuring it can be unreliable and less helpful.

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 diseases.

Interpreting thyroid function test results

While normal ranges may vary from lab to lab, generally each hormone should fall within a reference range that is considered normal. Acceptable ranges are:

• TSH: 0.4-4.5 mIU/mL

• T4:

  • Total: 4.5-12.5 ug/d

  • Free: 0.8-2.0 ng/dL

• T3:

  • Total: 80-215 ng/dL

  • Free: 1.5-6.0 pg/mL

• TPO Ab: <34 IU/mL

• Tg Ab: <1.0 IU/mL

Some institutions may use narrower ranges depending on how the blood is analyzed in the lab. When looking at your results, typically there is a reference range listed so you know if the measurement is normal for that specific lab.

If your results are outside the normal range, this may indicate temporary fluctuations in thyroid function rather than a longer-term thyroid condition. And day-to-day variation in levels can be normal and part of a healthy, responsive endocrine system.

What can affect thyroid test results?

Many factors may affect thyroid test results. And, so, 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 a woman’s pregnancy. So much so that 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 estrogen supplementation — which are often found in birth control pills — can falsely elevate total T3 and T4 levels.

• Biotin supplements: Some people take this for skin and hair health, and it is known to falsely decrease your TSH level. It is also found in most multivitamins, but the level of biotin is too small to make an impact.

Be sure to discuss any abnormal results with your provider. They can help you figure out what your results mean for you and your health, or even if they should be repeated on a different day.