Thyroid function tests are used to check for a number of thyroid problems. The tests measure the amount of different hormones including thyroid stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3). Other tests on the thyroid include various antibodies related to thyroid tissue.
TSH is produced by the pituitary gland in the brain. TSH is carried in the blood to the thyroid gland, which sits at the front of the neck. From there, TSH influences how much T4 and T3 are released into the blood.
TSH, T3 and T4 levels can all be tested in the blood. Your doctor will probably test TSH first, then T3 and T4 if TSH is abnormal. Tests can also be used to measure antibodies to diagnose autoimmune thyroid disease.
Why would I need these tests?
The thyroid gland's hormones help control some of your body's metabolic processes, such as heart rate, blood pressure, body temperature and weight. Too much or too little of these hormones can make you ill.
You might need thyroid tests if:
- you have symptoms of hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid)
- you are taking some form of thyroid hormone replacement treatment
- you are female and being investigated for infertility
Very rarely, babies are born without a working thyroid gland. For this reason, all Australian newborns are screened for hypothyroidism with a TSH test using a drop of blood taken from a heel prick.
How to prepare for this test
No preparation is usually needed for this test. In many cases, it can be done at any time of the day without fasting, but do check with your doctor ahead of time. Also let your doctor know if you are taking any medications before the test.
Understanding your results
A high TSH result might mean:
- hypothyroidism
- that you are receiving too little thyroid hormone medication to treat hypothyroidism
- there might be a problem with your pituitary gland
A low TSH result might mean:
- hyperthyroidism
- you are taking too much thyroid hormone medication to treat hypothyroidism
Your doctor will be aware of your circumstances and can discuss what the results mean for you.
More information
About thyroid function testing
Pathology Tests Explained has more information about TSH, T4 and T3 tests.
About blood testing
Visit our 'Guide to blood testing' to learn more about blood tests in general with information such as:
Guber HA, Oprea M, Russell YX. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 25.
Hinson J, Raven P. Endocrinology and the reproductive system. In: Niash J, Syndercombe D, eds. Medical Sciences. 3rd ed. Philadelphia, PA: Elsevier; 2019:chap 10.
Salvatore D, Cohen R, Kopp PA, Larsen PR. Thyroid pathophysiology and diagnostic evaluation. In: Melmed S, Auchus RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 11.
Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.
Last reviewed on: 1/9/2022
Reviewed by: Robert Hurd, MD, Professor of Endocrinology and Health Care Ethics, Xavier University, Cincinnati, OH. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
A simple, regular blood test is used to guide treatment for hypothyroidism and monitor the condition. Here’s what the numbers mean.
By Beth W. OrensteinMedically Reviewed by Kacy Church, MD
Reviewed: January 30, 2020
Medically Reviewed
If there’s a bright side to hypothyroidism, or an underactive thyroid, it’s that treatment usually just involves taking daily medication, and testing is limited to a simple blood test. Your hypothyroidism test results are your doctor’s blueprint for prescribing the right dose of synthetic thyroid hormone medication and tracking how well it’s working.
The main job of the thyroid gland is to make the hormone thyroxine, also known as T4 because it has four iodine molecules. The thyroid also makes the hormone triiodothyronine, known as T3 because it has three iodine molecules, but in smaller amounts, explains Cathy Doria-Medina, MD, an endocrinologist with HealthCare Partners Medical Group in Torrance, California. “The thyroid gland makes mostly T4, [and] the T4 has to be converted to T3, because T3 is the part of thyroxine that actually does the work,” she says. The pituitary gland at the base of the brain controls hormone production in your body. It makes thyroid-stimulating hormone (TSH), which tells the thyroid gland how much T4 and T3 to produce. The TSH level in your blood reveals how much T4 your pituitary gland is asking your thyroid gland to make. If your TSH levels are abnormally high, it could mean you have an underactive thyroid, or hypothyroidism. That’s because it indicates your pituitary gland is producing more TSH in an effort to stimulate your thyroid to produce thyroid hormone, according to the Mayo Clinic. “TSH levels go in the opposite direction of your thyroid hormone,” Dr. Doria-Medina explains. “If you’re making too little thyroid hormone, your TSH will go up. If you’re making too much thyroid hormone, your TSH will go down.” What’s normal can vary depending on a number of factors, including the laboratory where your blood test is done, she adds. A normal range for TSH in most laboratories is 0.4 milliunits per liter (mU/L) to 4.0 mU/L. If your TSH is higher than 4.0 mU/L on repeat tests, you probably have hypothyroidism. Your doctor may also order a T4 test. Most of the T4 in your blood attaches to a protein, and when it does, it can’t get into your cells. Only T4 that is unattached or “free” can get into your cells to go to work. A blood test can measure how much free T4 if available. Hypothyroidism is treated with daily medication. Taking synthetic thyroid hormone medication can bring your T4 and TSH levels back to their normal ranges. Once you’re on the right dose, your symptoms should subside. When you first start taking medication, your doctor will need to monitor your blood to fine-tune the dosage. “Initially you will need to be tested more frequently,” Doria-Medina says. “A person who is newly diagnosed and taking medication for hypothyroidism should be tested every six weeks until the dosage is just right.” The dose you start with is your doctor’s educated guess about what’s best for you — most likely the lowest dose possible to avoid side effects, which can include a rapid heartbeat and restlessness. Medication for hypothyroidism is slow acting, and it can take several weeks for your body to adjust. If your TSH is still high and your symptoms haven’t subsided after 6 to 10 weeks, your doctor will likely increase the dose, and you’ll need your blood tested again after another 6 to 10 weeks. Because you’ll need to take thyroid medication every day for the rest of your life, even after the right dose is found, your hormone levels will be monitored regularly to be sure that your treatment is working properly. “Eventually, most people with hypothyroidism can just be seen yearly by their doctor,” Doria-Medina says. The American Thyroid Association recommends that you keep your TSH within a narrow range of 0.5 to 2.5 mU/L, but don’t be alarmed if your test results vary a little. Some variation is normal because your pituitary gland sends out TSH in pulses, not a steady stream. Also, factors like the time of day you’re tested can make a difference. TSH levels are likely to be higher at night and lower during the day. Some people, including those who are pregnant or those with a history of thyroid cancer, have different TSH goals. Talk to your doctor about the target TSH range that’s right for you. If you have new or worsening symptoms or your health status changes — such as if you become pregnant, go through menopause, or are given another medicine that can interfere with the absorption of your thyroid hormones, such as anticonvulsants, certain antacids, or iron or calcium supplements — you should see your doctor and have your blood tested again, even if it’s ahead of schedule.Thyroxine, Triiodothyronine, and TSH Levels
Hypothyroidism Tests: A Measure of Treatment Success
How to Keep Hypothyroidism Under Control
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