Definition
Thyroid function tests are blood tests used to evaluate how effectively the thyroid gland is working. These tests include the thyroid-stimulating hormone test (TSH), the thyroxine test (T4), the triiodothyronine test (T3), the thyroxine-binding globulin test (TBG), the triiodothyronine resin uptake test (T3RU), and the long-acting thyroid stimulator test (LATS).
Purpose
Thyroid function tests are used to:
- Help diagnose an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism)
- Evaluate thyroid gland activity
- Monitor response to thyroid therapy.
Precautions
Thyroid treatment must be stopped one month before blood is drawn for a thyroxine (T4) test.
Steroids, propranolol (Inderal), cholestryamine (Questran), and other medications that may influence thyroid activity are usually stopped before a triiodothyronine (T3) test.
Estrogens, anabolic steroids, phenytoin, and thyroid medications may be discontinued prior to a thyroxine-binding globulin (TBG) test. The laboratory analyzing the blood sample must be told if the patient cannot stop taking any of these medications. Some patients will be told to take these medications as usual so that the doctor can determine how they affect thyroxine-binding globulin.
Patients are asked not to take estrogens, androgens, phenytoin (Dilantin), salicylates, and thyroid medications before having a triiodothyronine resin uptake (T3RU) test.
Prior to taking a long-acting thyroid stimulant (LATS) test, the patient will probably be told to stop taking all drugs that could affect test results.
Description
Most doctors consider the sensitive thyroid-stimulating hormone (TSH) test to be the most accurate measure of thyroid activity. By measuring the level of TSH, doctors can determine even small problems in thyroid activity. Because this test is sensitive, abnormalities in thyroid function can be determined before a patient complains of symptoms.
TSH "tells" the thyroid gland to secrete the hormones thyroxine (T4) and triiodothyronine (T3). Before TSH tests were used, standard blood tests measured levels of T4 and T3 to determine if the thyroid gland was working properly. The triiodothyrine (T3) test measures the amount of this hormone in the blood. T3 is normally present in very small amounts, but has a significant impact on metabolism. It is the active component of thyroid hormone.
The thyroxine-binding globulin (TBG) test measures blood levels of this substance, which is manufactured in the liver. TBG binds to T3 and T4, prevents the kidneys from flushing the hormones from the blood, and releases them when and where they are needed to regulate body functions.
The triiodothyronine resin uptake (T3RU) test measures blood T4 levels. Laboratory analysis of this test takes several days, and it is used less often than tests whose results are available more quickly.
The long-acting thyroid stimulator (LATS) test shows whether blood contains long-acting thyroid stimulator. Not normally present in blood, LATS causes the thyroid to produce and secrete abnormally high amounts of hormones.
It takes only minutes for a nurse or medical technician to collect the blood needed for these blood tests. A needle is inserted into a vein, usually in the forearm, and a small amount of blood is collected and sent to a laboratory for testing. The patient will usually feel minor discomfort from the "stick" of the needle.
Preparation
There is no need to make any changes in diet or activities. The patient may be asked to stop taking certain medications until after the test is performed.
Aftercare
Warm compresses can be used to relieve swelling or discomfort at the site of the puncture. With a doctor's approval, the patient may start taking medications stopped before the test.
Normal results
Not all laboratories measure or record thyroid hormone levels the same way. Each laboratory will provide a range of values that are considered normal for each test. Some acceptable ranges are listed below.
TSH
Normal TSH levels for adults are 0.5-5.0 mU/L.
T4
Normal T4 levels are:
- 10.1-2.0 ug/dl at birth
- 7.5-16.5 ug/dl at 1-4 months
- 5.5-14.5 ug/dl at 4-12 months
- 5.6-12.6 ug/dl at 1-6 years
- 4.9-11.7 ug/dl at 6-10 years
- 4-11 ug/dl at 10 years and older.
Levels of free T4 (thyroxine not attached to TBG) are higher in teenagers than in adults.
Normal T4 levels do not necessarily indicate normal thyroid function. T4 levels can register within normal ranges in a patient who:
- Is pregnant
- Has recently had contrast x rays
- Has nephrosis or cirrhosis.
T3
Normal T3 levels are:
- 90-170 ng/dl at birth
- 115-190 ng/dl at 6-12 years
- 110-230 ng/dl in adulthood.
TBG
Normal TBG levels are:
- 1.5-3.4 mg/dl or 15-34 mg/L in adults
- 2.9-5.4 mg/dl or 29-54 mg/L in children.
T3RU
Between 25-35% of T3 should bind to or be absorbed by the resin added to the blood sample. The test indirectly measures the amount of thyroid binding globulin (TBG) and thyroid-binding prealbumin (TBPA) in the blood.
LATS
Long-acting thyroid stimulator is found in the blood of only 5% of healthy people.
Abnormal results
T4
Elevated T4 levels can be caused by:
- Acute thyroiditis
- Birth control pills
- Clofibrate (Altromed-S)
- Contrast x rays using iodine
- Estrogen therapy
- Heparin
- Heroin
- Hyperthyroidism
- Pregnancy
- Thyrotoxicosis
- Toxic thyroid adenoma.
Cirrhosis and severe non-thyroid disease can raise T4 levels slightly.
Reduced T4 levels can be caused by:
- Anabolic steroids
- Androgens
- Antithyroid drugs
- Cretinism
- Hypothyroidism
- Kidney failure
- Lithium (Lithane, Lithonate)
- Myxedema
- Phenytoin
- Propranolol.
T3
Although T3 levels usually rise and fall when T4 levels do, T3 toxicosis causes T3 levels to rise while T4 levels remain normal. T3 toxicosis is a complication of:
- Graves' disease
- Toxic adenoma
- Toxic nodular goiter.
T3 levels normally rise when a woman is pregnant or using birth-control pills. Elevated T3 levels can also occur in patients who use estrogen or methadone or who have:
- Certain genetic disorders that do not involve thyroid malfunction
- Hyperthyroidism
- Thyroiditis
- T3 thyrotoxicosis
- Toxic adenoma.
Low T3 levels may be a symptom of:
- Acute or chronic illness
- Hypothyroidism
- Kidney or liver disease
- Starvation.
Decreased T3 levels can also be caused by using:
- Anabolic steroids
- Androgens
- Phenytoin
- Propranolol
- Reserpine (Serpasil)
- Salicylates in high doses.
TBG
TBG levels, normally high during pregnancy, are also high in newborns. Elevated TBG levels can also be symptoms of:
- Acute hepatitis
- Acute intermittent porphyria
- Hypothyroidism
- Inherited thyroid hormone abnormality.
TBG levels can also become high by using:
- Anabolic steroids
- Birth control pills
- Anti-thyroid agents
- Clofibrate
- Estrogen therapy
- Phenytoin
- Salicylates in high doses
- Thiazides
- Thyroid medications
- Warfarin (Coumadin).
TBG levels can be raised or lowered by inherited liver disease whose cause is unknown.
Low TBG levels can be a symptom of:
- Acromegaly
- Acute hepatitis or other acute illness
- Hyperthyroidism
- Kidney disease
- Malnutrition
- Marked hypoproteinemia
- Uncompensated acidosis.
T3RU
A high degree of resin uptake and high thyroxine levels indicate hyperthyroidism. A low degree of resin uptake, coupled with low thyroxine levels, is a symptom of hypothyroidism.
Thyroxine and triiodothyronine resin uptake that are not both high or low may be a symptom of a thyroxine-binding abnormality.
LATS
Long-acting thyroid stimulator, not usually found in blood, is present in the blood of 80% of patients with Graves' disease. It is a symptom of this disease whether or not symptoms of hyperthyroidism are detected.
Key Terms
- Acidosis
- A condition in which blood and tissues are unusually acidic.
- Acromegaly
- A disorder in which growth hormone (a chemical released from the pituitary gland in the brain) causes increased growth in bone and soft tissue. Patients have enlarged hands, feet, noses, and ears, as well as a variety of other disturbances throughout the body.
- Acute intermittent porphyria
- An inherited disease affecting the liver and bone marrow. The liver overproduces a specific acid and the disease is characterized by attacks of high blood pressure, abdominal colic, psychosis, and nervous system disorders.
- Anabolic steroids
- Protein-building compounds used to treat certain anemias and cancers, strengthen bones, and stimulate weight gain and growth. Anabolic steroids are sometimes used to illegally enhance athletic performance.
- Cholestryamine (Questran)
- A drug used to bind with bile acids and prevent their reabsorption and to stimulate fat absorption.
- Cirrhosis
- Progressive disease of the liver, associated with failure in liver cell functioning and blood flow in the liver. Tissue and cells are damaged, the liver becomes fibrous, and jaundice can result.
- Clofibrate (Altromed-S)
- Medication used to lower levels of blood cholesterol and triglycerides.
- Cretinism
- Severe hypothyroidism that is present at birth and characterized by severe mental retardation.
- Graves' disease
- The most common form of hyperthyroidism, characterized by bulging eyes, rapid heart rate, and other symptoms.
- Heparin
- An organic acid that occurs naturally in the body and prevents blood clots. Heparin is also made synthetically and can be given as a treatment when required.
- Hepatitis
- Inflammation of the liver.
- Hyperthyroidism
- Overactive thyroid gland; symptoms include irritability/nervousness, muscle weakness, tremors, irregular menstrual periods, weight loss, sleep problems, thyroid enlargement, heat sensitivity, and vision/eye problems. The most common type of this disorder is called Graves' disease.
- Hypoproteinemia
- Abnormally low levels of protein in the blood.
- Hypothyroidism
- Underactive thyroid gland; symptoms include fatigue, difficulty swallowing, mood swings, hoarse voice, sensitivity to cold, forgetfulness, and dry/coarse skin and hair.
- Lithium (Lithane, Lithromate)
- Medication prescribed to treat manic (excited) phases of bipolar disorder.
- Myxedema
- Hypothyroidism, characterized by thick, puffy features, an enlarged tongue, and lack of emotion.
- Nephrosis
- Any degenerative disease of the kidney (not to be confused with nephritis, an inflammation of the kidney due to bacteria).
- Nodular goiter
- An enlargement of the thyroid (goiter) caused when groups of cells collect to form nodules.
- Phenytoin (Dilantin)
- Anti-convulsive medication used to treat seizure disorders.
- Propranolol (Inderal)
- Medication commonly prescribed to treat high blood pressure; is a beta-adrenergic blocker and can also be used to treat irregular heartbeat, heart attack, migraine, and tremors.
- Reserpine (Serpasil)
- A drug prescribed for high blood pressure.
- Salicylates
- Aspirin and certain other nonsteroidal anti-inflammatory drugs (NSAIDs).
- Thiazides
- A group of drugs used to increase urine output.
- Thyroid gland
- A butterfly-shaped gland in front and to the sides of the upper part of the windpipe; influences body processes like growth, development, reproduction, and metabolism.
- Thyroiditis
- Inflammation of the thyroid gland.
- Thyrotoxicosis
- A condition resulting from high levels of thyroid hormones in the blood.
- Toxic thyroid adenoma
- Self-contained concentrations of thyroid tissue that may produce excessive amounts of thyroid hormone.
Further Reading
For Your Information
Books
- Fischbach, Frances Talaska. A Manual of Laboratory and Diagnostic Tests, 5th ed. Philadelphia, PA: J.B.Lippincott Co., 1996.
- Pagana, Kathleen Deska, and Timothy James Pagana. Mosby's Diagnostic and Laboratory Test Reference, 3rd ed. St. Louis, MO: Mosby-Year Book, Inc., 1997.
- Shaw, Michael, ed. Everything You Need to Know About Medical Tests. Springhouse, PA: Springhouse Corp., 1996.
Organizations
- The American Thyroid Association, Inc. Montefiore Medical Center, 111 E. 210th St., Bronx, NY 10467. http://www.thyroid.org.
- The Thyroid Foundation of America, Inc. Ruth Sleeper Hall, RSL350, 40 Parkman St., Boston, MA 02114-2698. (800) 832-8321. http://www.tfaeweb.org/pub/tfa.
Gale Encyclopedia of Medicine. Gale Research, 1999.
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