Also called: T4 Test, TSH Test, T3 Test
Thyroid blood tests indicate how well a person’s thyroid gland is functioning. The thyroid produces hormones that are essential to regulating the body’s metabolism, the physical and chemical processes necessary for the maintenance of life. These blood tests may help physicians diagnose conditions that relate to the thyroid gland, such as forms of hyperthyroidism (e.g., Graves’ disease) and hypothyroidism.
The major thyroid blood tests include:
- TSH test. Normally, this the first test of thyroid function to be performed. It measures levels of thyroid-stimulating hormone (TSH), a substance that regulates the thyroid gland.
- T4 test. Measures levels of the hormone thyroxine (T4), which circulates in the body either bound to proteins or freely on its own.
- T3 test. Measures levels of the hormone triiodothyronine (T3). As with T4, T3 circulates in the body either attached to proteins or freely circulating on its own.
- Autoantibody test. Some patients with thyroid disorders have antibodies that affect the thyroid gland, causing it to produce either too much or too little thyroid hormone. This test can reveal the presence of those autoantibodies.
- Serum TBG. Measures the level of a protein called thyroxine binding globulin (TBG) in the blood.
The use of various medications, including aspirin, birth control pills and thyroid medication, can affect the outcome of the tests. Certain medical conditions may also affect results. Patients should inform their physician of any medications they may be taking as well as whether they are ill or may be pregnant.
Patients should discuss the results of their thyroid blood tests with a physician. In some cases other diagnostic tests may be required, such as imaging tests or biopsies.
About thyroid blood tests
Thyroid blood tests are used to measure the health of the thyroid, a butterfly-shaped gland at the base of the neck. The thyroid secretes hormones involved in regulating a person’s metabolism, the physical and chemical processes necessary for the maintenance of life. Thyroid blood tests are important tools to screen for and monitor thyroid disease. There are two major forms of thyroid disease:
- Hypothyroidism, characterized by low thyroid levels
- Hyperthyroidism, characterized by high thyroid levels
The thyroid gland produces two major hormones: thyroxine (T4) and triiodothyronine (T3). Thyroxine makes up more than 90 percent of the hormone produced by the thyroid. Various thyroid disorders can cause the thyroid gland to release too much or too little T4 or T3, resulting in a range of symptoms
Another hormone called thyroid-stimulating hormone (TSH) causes the thyroid gland to manufacture thyroxine. Levels of TSH may also be measured to help physicians diagnose thyroid problems.
The major thyroid blood tests include:
- TSH test. If a physician suspects a thyroid problem, a TSH test is usually the first test to be performed. High levels of TSH typically are present when the patient’s thyroid is failing because of a disorder that directly affects the thyroid (primary hypothyroidism). When TSH levels are low, it usually indicates that the thyroid gland is overactive (hyperthyroidism), although it also can signal a problem with the pituitary gland (secondary hypothyroidism).
- T4 test. T4 circulates in the body either bound to proteins or freely circulating on its own. Although freely circulating T4 makes up just 0.1 percent of total T4, free T4 is the part of the hormone that penetrates target tissues and delivers its effects. T4 testing can be used to measure both bound and free T4 (total T4 test). However, a newer test allows a narrower measurement of just the circulating levels of T4 (free T4 test). This free T4 (FT4) test generally is viewed as more accurate than total T4 testing.
- T3 test. As with T4, T3 circulates in the body either attached to proteins (about 99.7 percent of T3 in the body) or freely circulating on its own. A T3 test may be performed to diagnose hyperthyroidism, with elevated levels of T3 indicating the condition. Typically, this test is performed after abnormal results are reported in TSH and T4 testing. T3 testing is usually not helpful in diagnosing hypothyroidism. T3 testing can be used to measure either all T3 in the body (total T3 test) or just circulating levels of T3 (free T3 test).
- Serum TBG level. This test measures the level of a protein called thyroxine binding globulin (TBG) in the blood. TBG transports thyroid hormone throughout the body. The test can be used to identify abnormalities in a patient’s TGB or to help indicate conditions including thyroid disorders, kidney disease and acromegaly.
- Autoantibody test. Normally, the immune system produces antibodies to protect the body against diseases. However, patients with autoimmune disorders have immune systems that mistakenly produce autoantibodies (antibodies that attack healthy tissue). Some patients with hypothyroidism or hyperthyroidism have antibodies that attack the thyroid gland, causing it to either produce too much or too little thyroid hormone.
Other thyroid blood tests may also be used to help physicians diagnose thyroid problems. For example, a TRH test, in which a physician injects TRH (TSH-releasing hormone) into the patient, can help reveal if hypothyroidism is due to a problem with the pituitary gland or hypothalamus. Improved TSH testing has largely replaced the TRH test.
Additionally, a procedure known as a TSI test measures for thyroid-stimulating immunoglobulin (TSI), a substance often found in the blood of patients with Graves’ disease. However, TSI tests are rarely performed because the information they provide usually is not necessary to formulate a treatment plan.
If thyroid cancer is suspected, a combination of thyroid blood tests may be conducted. These may include a TSH test to assess the condition of the gland and a calcitonin test to help confirm or rule out a type of cancer called medullary thyroid carcinoma. Scientists have found that a newer blood test for a substance called TSH receptor message ribonucleic acid (TSHR mRNA) can also indicate thyroid cancer.
Thyroid blood tests may be recommended by physicians for a number of reasons. They are most often performed when a physician suspects that a patient’s symptoms are related to a thyroid disorder. They also may be performed to monitor the health of patients previously diagnosed with thyroid conditions.
Additionally, in the United States, TSH and T4 testing is routinely performed on newborns to screen for congenital hypothyroidism, a condition that can cause mental retardation if left untreated. A combination of TSH and T4 testing may also be used to diagnose and monitor female infertility problems and to diagnose pituitary problems.
Some experts have suggested that adults over the age of 35 should undergo routine thyroid blood testing. Others organizations recommend screening for adults over age 60 or for women who are 50 and older. To date, there is no consensus on the need for routine screening.
Depending on thyroid blood test results, a physician may recommend additional assessments, including thyroid imaging tests such as a thyroid scan or an ultrasound.
Factors that may affect test results
There are various factors that may affect thyroid blood test results. For example, thyroid-stimulating hormone (TSH) test results may be affected by certain medications. These include antithyroid medications, lithium, potassium iodide, dopamine, amiodarone (an antiarrhythmic heart drug) and prednisone (a corticosteroid).
Thyroxine (T4) and triiodothyronine (T3) levels also may be affected by medications, leading to potentially inaccurate test results. Medications that may raise or lower T4 and T3 levels include:
- Anabolic steroids
- Antithyroid medication
- Birth control pills
- Aspirin, meclofenamate or salsalate (nonsteroidal anti-inflammatory drugs)
- Clofibrate (a cholesterol drug)
- Methadone (an opioid)
- Phenytoin (an anticonvulsant)
- Propranolol (a beta blocker – see Antihypertensives)
Certain medications may affect other types of thyroid blood tests. For example, phenothiazines (antipsychotics), heroin and methadone increase levels of thyroxine binding globulin (TBG) in the blood. This may lead to inaccurate serum TBG tests. Patients should inform their physician of any medications they are taking before undergoing thyroid blood testing.
In addition to medications, certain health conditions can affect thyroid blood test results. For example, patients who have a chronic or systemic (throughout the body) illness typically have lower T4 and T3 levels. These patients are advised to wait until they are healthy before undergoing thyroid blood testing. Additionally, patients who are pregnant may have higher T4 or T3 levels.
Conditions that may affect TBG results include severe liver or kidney disease, acute intermittent porphyria (inherited metabolic disorder), human immunodeficiency virus (HIV), as well as other conditions that lower levels of albumin.
Understanding thyroid blood test results
The results of thyroid blood tests offer information about how well or poorly a patient’s thyroid gland is functioning. Though general normal ranges have been established, these ranges tend to vary from laboratory to laboratory. The numbers that follow should be used as a guide and should not be considered definitive. Patients should consult their physician about the implications of their test results.
If a physician suspects a thyroid problem, a TSH (thyroid-stimulating hormone) test is usually the first test to be performed. Normal results for TSH testing differ depending on whether the patient has a history of thyroid problems.
Normal levels of TSH for patients with no history of thyroid problems are 0.4 to 4.0 milli-international units per liter (mIU/L). However, patients who have no history of a thyroid problem but who have a reading of 2.5 mIU/L or higher may develop hypothyroidism in the future and should be monitored closely. Normal levels for patients who do have a history of thyroid problems are 0.5 to 2.5 mIU/L.
TSH readings that are higher than normal may indicate any of the following:
- Congenital hypothyroidism
- Primary hypothyroidism
- TSH-dependent hyperthyroidism
- Thyroid hormone resistance
- Exposure to mice
TSH readings that are lower than normal may indicate any of the following:
- TSH deficiency
- Use of certain medications (e.g., dopamine agonists, corticosteroids, somatostatin analogues, bexarotene)
A thyroxine (T4) test may be conducted in conjunction with the TSH test. Generally, normal results for T4 testing are between 4.5 to 11.2 micrograms per deciliter (mcg/dL). Higher than average T4 levels in addition to lower than average TSH levels may indicate a form of hyperthyroidism, such as:
- Graves’ disease
- Toxic multinodular goiter
- Subacute or chronic thyroiditis
- Early Hashimoto’s disease
- Iodine-induced hyperthyroidism
- Germ cell tumors (found mainly in the ovaries and testicles)
- Trophoblastic disease (type of uterine cancer)
Levels of T4 that are lower than normal may indicate hypothyroidism, or any of the following:
- Systemic (throughout the body) illness
- Use of certain medications (e.g., dexamethasone, propranolol, lithium, iodine, methimazole, propylthiouracil, interferon alfa, interleukin-2 and amiodarone)
A triiodothyronine (T3) test may be performed if abnormal results are reported in TSH and T4 testing. Generally, normal results for T3 testing are between 100 to 200 nanograms per deciliter (ng/dL). Greater than normal T3 levels in addition to low TSH levels may indicate hyperthyroidism. In rare cases, this also may indicate thyroid cancer. Tests results in which T3 levels are lower than normal may indicate hypothyroidism, chronic illness or starvation.
Results from the three main types of thyroid blood tests may be combined to arrive at a diagnosis. Such results are as follows:
|T3 and T4 normal, TSH high||Mild hypothyroidism|
|T3 low or normal, T4 low, TSH high||Hypothyroidism|
|T3 and T4 normal, TSH low||Mild hyperthyroidism|
|T3 and T4 high or normal, TSH low||Hyperthyroidism|
|T3 and T4 low or normal, TSH low||Pituitary hypothyroidism|
Another test, called a serum TBG test, measures the level of a protein called thyroxine binding globulin (TBG) in the blood. Normal values for TBG testing differ depending on the method used to analyze a sample. When electrophoresis (the use of an electrical field to separate charged particles) is used, normal values range from 10 micrograms (mg)/100 milliliters (ml) to 25 mg/100 ml. When radioimmunoassay (procedure using small amounts of a radioactive substance) is used, normal levels range from 1.3 to 2.0 mg/100 ml.
Elevated levels of TBG are normal in newborns. However, in adults elevated levels of TBG may indicate hypothyroidism and other problems with the thyroid gland. It also may indicate:
- Liver disease
- Acute intermittent porphyria (an inherited disorder of metabolism)
TBG levels that are lower than normal may indicate hyperthyroidism or any of the following:
- Acromegaly (disorder caused by overactive pituitary gland)
- Acute illness
- Stress associated with surgery
- Nephrotic syndrome (a disorder, sometimes caused by diabetic nephropathy, that involves proteinuria, high cholesterol and edema)
In some cases, high or low levels of TBG are due to an inherited trait and are not signs of any disorder.
Questions for your doctor on thyroid blood tests
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about thyroid blood tests:
- What type of thyroid blood test will you be performing?
- What conditions can you detect with this thyroid blood test? What do you suspect I have?
- Will I require more than one type of thyroid blood test?
- Can I eat before the test or do I have to fast for a certain period of time beforehand?
- How else should I prepare for the test?
- How many vials of my blood are needed for this test?
- Are there any significant risks associated with the test?
- Does the test have any side effects? Which should I report to you?
- How long will I have to wait for test results?
- Will I need additional diagnostic testing to confirm the thyroid blood test results? What type?
- If don’t have any signs of illness, should I be screened with a thyroid blood test?