What Are Thyroid Function Tests?

The thyroid is a small gland, shaped like a butterfly, which is located in the lower-front part of your neck. Your doctor can feel your thyroid by pressing his or her fingers gently against your throat. The thyroid is responsible for helping to regulate many of the body’s processes, such as metabolism, energy generation, and mood.

The thyroid produces two major hormones: triiodothyronine (T3) and thyroxine (T4). If your thyroid gland does not 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.”

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.

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, the doctor will order further tests to pinpoint the reason for the problem.

Part 2 of 5: Procedure

Drawing Blood for Thyroid Function Tests

Talk to your doctor about any medications you are taking and tell your doctor if you are pregnant. Certain medications and pregnancy may influence your test results.

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

A technician (or nurse) will tie a band of rubber tightly around your upper arm to make the veins swell with blood. Once the technician has found an appropriate vein, he or she will insert a needle under the skin and into the vein. You may feel a sharp prick when the needle punctures the skin. The technician will collect blood in test tubes and send it to a laboratory for analysis.

When the technician has gathered the amount of blood needed for the test or tests, he or she will withdraw the needle and place pressure on the puncture wound until the bleeding stops. The technician will then place a small bandage over the wound.

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

Part 3 of 5: Side Effects

Side Effects and Aftercare

Venipuncture is a routine, minimally invasive procedure. During the days immediately after the blood test, you may notice slight bruising or soreness at the area where the needle went in. An ice pack or 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.

Part 4 of 5: Test Results

Understanding Your Test Results

Free T4 and TSH Results

The T4 test is known as the thyroxine test. The TSH test measures the level of thyroid-stimulating hormone in your blood. These are the two most common thyroid function tests and are usually ordered together.

They 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. When adults suffer from hypothyroidism, they experience weight gain, fatigue, depression, and brittle hair and fingernails.

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 2.0 mIU/L, you are at risk for progressing to full-blown hypothyroidism. Your doctor will likely want to perform thyroid function tests at least every other year going forward. Your doctor may also decide to begin treating you with medications, such as levothyroxine, to ease your symptoms.

A high level of T4 indicates an overactive thyroid (hyperthyroidism). Symptoms include anxiety, unplanned weight loss, tremors, and diarrhea.

T3 Results

T3 tests, which check for levels of triiodothyronine, are usually ordered if T4 tests and TSH tests suggest hyperthyroidism. The T3 test may also be ordered if you are showing signs of an overactive thyroid gland.

The normal range for the T3 is 100-200 nanograms of hormone per deciliter of blood. Abnormally high levels most commonly indicate a high amount of estrogen in the blood or a condition called Grave’s disease. This is an autoimmune disorder associated with hyperthyroidism.

T3 Resin Uptake Results

A T3 resin uptake, also known as a T3RU, is a blood test that measures a hormone called thyroxin-binding globulin (TBG). If your T3 level is elevated, your TBG level should be low.

Abnormally high levels of TBG often indicate a problem with the kidneys or with the body not getting enough protein. Abnormally low levels of TBG suggest high levels of estrogen in the body, which may be caused by pregnancy; eating estrogen-rich foods, such as flaxseed; obesity; or hormone replacement therapy.

Part 5 of 5: Follow-Up


If your blood work suggests that your thyroid gland is overactive or underactive, your doctor may order a thyroid uptake test or an ultrasound test to ensure that the problem is not due to a structural problem with the thyroid gland or a tumor.

If the scan is normal, your doctor will probably prescribe medication to regulate your thyroid activity and will follow up with additional thyroid function tests to make sure the medicine is working.

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