image shows an x-ray type image of a person's neck in blue, with the thyroid isolated as red so it is easier to see.Share on Pinterest
The thyroid is a butterfly shape and is shown here in red.

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Thyroid function tests are a series of blood tests used to measure how well your thyroid gland is working. Available tests include the T3, T3RU, T4, and TSH.

The thyroid is a small gland located in the lower-front part of your neck. It’s responsible for helping regulate many bodily processes, such as metabolism, energy generation, and mood.

The thyroid produces two major hormones: triiodothyronine (T3) and thyroxine (T4). If your thyroid gland doesn’t produce enough of these hormones, you may experience symptoms such as weight gain, lack of energy, and depression. This condition is called hypothyroidism.

If your thyroid gland produces too many hormones, you may experience weight loss, high levels of anxiety, tremors, and a sense of being on a “high.” This is called hyperthyroidism.

Typically, a doctor who is concerned about your thyroid hormone levels will order broad screening tests, such as the T4 or the thyroid-stimulating hormone (TSH) test. If those results come back abnormal, your doctor will order further tests to pinpoint the reason for the problem.

If you’re concerned about your thyroid function and don’t already have a primary care doctor, you can view doctors in your area through the Healthline FindCare tool.

There are also at-home thyroid function tests available from LetsGetChecked and Everlywell. Healthline has also created a list of best at-home thyroid tests here.

You don’t need to do anything special to prepare for the thyroid function tests.

If a healthcare professional has ordered other blood tests to be taken at the same time, you may need to fast for several hours before the test. They will let you know of any special instructions to follow.

Otherwise, you will not need to follow any specific directions before the test.

Before you get a blood draw to check your thyroid levels, talk with your doctor about any medications you’re taking. Also let them know if you’re pregnant. Certain medications and being pregnant may influence your test results.

A blood draw, also known as venipuncture, is a procedure performed at a lab or a doctor’s office. When you arrive for the test, you’ll be asked to sit in a comfortable chair or lie down on a cot or gurney. If you’re wearing long sleeves, you’ll be asked to roll up one sleeve or to remove your arm from the sleeve.

A healthcare professional, like a technician or nurse, will tie a band of rubber tightly around your upper arm to make the veins swell with blood. Once the healthcare professional has found an appropriate vein, they’ll insert a needle under the skin and into the vein.

You may feel a sharp prick when the needle punctures your skin. The healthcare professional will collect your blood in test tubes and send it to a laboratory for analysis.

When the healthcare professional has gathered the amount of blood needed for the tests, they’ll withdraw the needle and place pressure on the puncture wound until the bleeding stops. They will then place a small bandage over the wound.

You should be able to return to your typical daily activities immediately.

A blood draw is a routine, minimally invasive procedure and doesn’t have many side effects.

During the days immediately after the blood draw, you may notice slight bruising or soreness at the area where the needle was inserted. Placing an ice pack on the affected site or taking an over-the-counter pain reliever can help ease your discomfort.

If you experience a great deal of pain, or if the area around the puncture becomes red and swollen, follow up with your doctor immediately. These could be signs of an infection.

T4 and TSH results

The TSH test and the T4 test are the two most common thyroid function tests. The TSH test is often done first because it’s the best way to initially test thyroid function. It determines whether a person has hyperthyroidism or hypothyroidism.

The T4 test is known as the thyroxine test. A high level of T4 indicates an overactive thyroid (hyperthyroidism). Symptoms include:

  • anxiety
  • unplanned weight loss
  • tremors
  • diarrhea

Most of the T4 in your body is bound to protein. A small portion of T4 is not, and this is called free T4.

Free T4 is the form that is readily available for your body to use. Sometimes a free T4 level is also checked along with the T4 test.

The TSH test measures the level of thyroid-stimulating hormone in your blood. The TSH has a normal test range between 0.4 and 4.0 milli-international units of hormone per liter of blood (mIU/L).

If you show signs of hypothyroidism and have a TSH reading above 4.5 mIU/L, you’re at risk of progressing to hypothyroidism. Symptoms can include:

Your doctor may decide to begin treating your symptoms with medications, such as levothyroxine.

Both the T4 and TSH tests are routinely performed on newborn babies to identify a low functioning thyroid gland. If left untreated, this condition, called congenital hypothyroidism, can lead to developmental disabilities.

T3 results

The T3 test checks for levels of the hormone triiodothyronine. It’s usually ordered if T4 tests show elevation.

Abnormally high levels of T3 most commonly indicate a condition called Grave’s disease. It’s an autoimmune disorder associated with hyperthyroidism.

If your blood work suggests your thyroid gland is overactive, your doctor may order a thyroid uptake test or an ultrasound.

If scan results are normal, your doctor will likely prescribe medication to regulate your thyroid activity. They will follow up with additional thyroid function tests to make sure the medication is working. These tests will check for:

  • structural problems with the thyroid gland
  • thyroid gland activity
  • any tumors that may be causing problems

Your doctor may also order an ultrasound if they discover abnormal findings during a physical exam of your neck. If ultrasound results are abnormal, your doctor may order a tissue sample of the thyroid.

Thyroid cancer is not related to hyperthyroid or hypothyroid. Keep in mind that blood tests are not used to find thyroid cancer.