Thyroid Hormone Blood Test

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KEY POINTS

• A thyroid hormone blood test measures the level of thyroid hormones in your blood. Thyroid hormones help control some of our body functions such as temperature, heart rate, and appetite.
• This test can help show if your thyroid gland is making too many or too few thyroid hormones. If you take thyroid hormone medicine, this test also helps to check that you are taking the right dose.
• Talk to your healthcare provider about what the test results mean and ask any questions you have.

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What is a thyroid hormone blood test?

A thyroid hormone blood test measures the level of thyroid hormones in your blood. Thyroid hormones are chemicals made by the thyroid gland. The thyroid gland is in the lower front of your neck. Thyroid hormones control the way your body turns the food you eat into energy. They also help control body functions such as temperature, heart rate, and appetite.

Why is this test done?

This test can help show whether your thyroid gland is making too much or too little thyroid hormone. You may have one or both of the two thyroid hormones checked with this test. The hormones are called thyroxine (T4) and triiodothyronine (T3). Your healthcare provider may also check your thyroid stimulating hormone (TSH) level.

If you take thyroid hormone medicine, this test also helps to check that you are taking the right dose.

How do I prepare for this test?

• Your healthcare provider will tell you if you need to stop eating and drinking before the test. Food and drink before the test usually do not affect the results.
• You may need to avoid taking certain medicines before the test because they might affect the test result. Make sure your healthcare provider knows about any medicines, herbs, or supplements that you are taking. Don't stop any of your regular medicines without first talking to your healthcare provider about it.

How is the test done?

Having this test will take just a few minutes. A small amount of blood is taken from a vein in your arm with a needle. The blood is collected in tubes and sent to a lab.

Ask your healthcare provider when and how you will get the result of your test.

What does the test result mean?

Your thyroid hormone levels may be higher than normal if:

• Your thyroid gland is making too many hormones.
• You are taking too much thyroid hormone medicine.

Your thyroid hormone levels may be lower than normal because:

• Your thyroid gland is not making enough thyroid hormones.
• You are not taking enough thyroid hormone medicine.

Other things can affect the level of thyroid hormone in your blood. For example:

• Results from this test may be abnormal when you are sick.
• Other hormones in the body may affect thyroid hormone levels.
• Some medicines may affect thyroid hormone levels.

Many things can affect thyroid hormone levels, so other thyroid function tests (a thyroid panel) may be done in addition to or instead of this test.

What if my test result is not normal?

Test results are only one part of a larger picture that takes into account your medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your healthcare provider about the result and ask questions, such as:

• If you need more tests
• What kind of treatment you might need
• What lifestyle, diet, or other changes you might need to make