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Thyroid Function Tests

Thyroid Function Tests

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), cholestyramine (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 one to four months
  • 5.5-14.5 ug/dl at four to 12 months
  • 5.6-12.6 ug/dl at one to six years
  • 4.9-11.7 ug/dl at 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 six to 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% and 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:

  • 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

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.

Cholestyramine (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.

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.

Resources

ORGANIZATIONS

American Thyroid Association, Inc. Montefiore Medical Center, 111 E. 210th St., Bronx, NY 10467. http://www.thyroid.org.

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.

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Thyroid Function Tests

THYROID FUNCTION TESTS

The key tests to determine thyroid function are serum measurements of free thyroid hormones and thyroid-stimulating hormone (TSH). Thyroid hormones have a negative feedback on TSH secretion from the anterior pituitary. In hyperthyroidism, free thyroid hormones are increased above the normal range and TSH levels are markedly decreased. In hypothyroidism, free thyroid hormones are decreased and TSH concentrations are increased when the cause is disease of the thyroid gland; when caused by a deficiency of TSH, free thyroid hormones are decreased but TSH is usually low. Radioactive iodine studies of the thyroid gland, which used to be the mainstay of testing, have been supplanted by these blood tests.

Martin I. Surks

(see also: Goiter; Hyperthyroidism; Hypothyroidism; Iodine; Thyroid Disorders )

Bibliography

Kaptein, E. M., and Nelson, J. C. (1999). "Serum Thyroid Hormones and Thyroid-Stimulating Hormone." In Atlas of Clinical Endocrinology, Vol. I: Thyroid Diseases, ed. M. I. Surks. Philadelphia, PA: Current Medicine.

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Thyroid Function Tests

Thyroid Function Tests

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), free and total thyroxine tests (FT4,T4), the free and total triiodothyronine tests (FT3,T3), the thyroxine-binding globulin test (TBG), and the T-uptake test.

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.

Thyroid hormones regulate the rate of cellular activity and affect body temperature, appetite, sleep, and mental health. A low level of thyroid hormone results in myxedema. Although the severity of disease may range from very mild to severe, symptoms associated with hypothyroidism are anemia, malaise, intolerance to cold, hyperlipidemia, fluid retention, and depression. A high level of thyroid hormone causes hyperthyroidism. Classical symptoms include insomnia, intolerance to heat, weight loss, and rapid heart rate.

Both hypoand hyperthyroidism can be caused by several mechanisms. Primary hypo- and hyperthyroidism are caused by conditions intrinsic to the thyroid, while secondary hypo- and hyperthyroidism are caused by pituitary-hypothalmic failure. T4 is present in much higher concentrations than T3, but T3 is physiologically more potent. Thyroid hormones are active only when not protein bound (i.e., as free hormone). Circulating free hormone levels are regulated by pituitary release of thyroid stimulating hormone (TSH). The release of TSH is controlled by negative feedback. Increased blood levels of free hormone inhibit pituitary release of TSH.

Precautions

Many drugs affect the results of thyroid function tests without causing thyroid disease. Some common drugs known to depress thyroid hormone levels are dopamine, corticosteroids, lithium, salicylates, anti-convulsants, and androgens. Thyroid hormone levels may be increased by estrogens, clofibrate, and opiates. TSH, TBG, and T-uptake levels are also affected by many of the drugs cited above. In addition, acute and chronic illnesses and pregnancy also affect thyroid function tests. Such conditions may be confused with clinical hypo- or hyperthyroidism. When possible, patients may be asked to discontinue medications that are known to interfere with the tests several days or more prior to testing.

While most drugs that interfere with thyroid function tests do so by altering thyroxine-binding protein concentrations, peripheral conversion of T4 to T3, and other in vivo mechanisms, a few substances (mainly heterophile and autoantibodies) may interfere directly with the analysis. Such interference should be suspected by a physician who sees a test result that is inconsistent with the patient's symptoms or other thyroid function test results.

Description

Currently, thyroid testing is performed on plasma or serum specimens using immunoassay methods including enzyme-multiplied immunoassay technique (EMIT), cloned enzyme donor immunoassay (CEDIA), radioimmunoassay (RIA), fluorescence polarization immunoassay (FPIA), and chemiluminescence.

The high-sensitivity thyroid-stimulating hormone (TSH) test is the most sensitive and specific screening test for thyroid disease. TSH levels change exponentially with changes in T4 and T3 and are less likely to be elevated or depressed by nonthyroid illnesses or drugs.

This strategy is more cost-effective than a panel approach (e.g., TSH + FT4 or FT4 +FT3) but necessitates the use of a TSH assay with a functional sensitivity below 0.02 mU/L. This level of sensitivity is required to differentiate primary hyperthyroidism, which causes levels to be near undetectable from the low end of the reference range, which is only 0.4 mU/L. A normal TSH level rules out clinical thyroid disease. Low TSH levels may result from primary hyperthyroidism or secondary hypothyroidism caused by pituitary TSH deficiency. High TSH levels are caused by primary hypothyroidism or secondary hyperthyroidism resulting from pituitary adenoma. Abnormal TSH levels are followed by measurements of T3 and T4 (preferably free T4) to confirm the diagnosis. For example, a person with a low TSH who has primary hyperthyroidism will have an elevated T3 and usually an elevated free T4; a person with a low TSH caused by pituitary disease will have low levels of these hormones. Measurement of T4 (and FT4) is considered a more specific indicator of hypothyroidism than T3, while T3 (and FT3) are more sensitive in detecting cases of hyperthyroidism than is T4.

TSH levels are sometimes abnormal in persons with subclinical thyroid disease and in patients with severe acute or chronic illness (called euthyroid sick syndrome). These cases may require the thyrotropin-releasing hormone stimulation test (TRH stimulation test) and reverse T3 test to determine if underlying thyroid disease is present. TRH stimulation is performed by measurement of the TSH level followed by IV administration of thyrotropin releasing factor. The TSH is measured 30 and 60 minutes after the injection. Persons with primary hypothyroidism show an excessive TSH response. The TRH stimulation test is usually normal in persons with euthyroid sick syndrome. Reverse T3 forms from peripheral conversion of T4 to T3. Levels of rT3 are low in persons with hypothyroidism and usually increased in persons with euthyroid sick syndrome.

Pregnancy, certain diseases (e.g., viral hepatitis), and several drugs (e.g., steroids) affect the level of thyroxine-binding proteins. In such cases, the level of total hormone will be abnormal, but the level of free hormone will be unaffected. FT4 and FT3 improve diagnostic accuracy for detecting hypo- and hyperthyroidism in patients with thyroid hormonebinding abnormalities that compromise the diagnostic utility of total hormone tests.

In cases where abnormal levels of thyroxinebinding proteins is suspected, two tests are helpful, the T-uptake test and measurement of thyroxine binding globulin (TBG). The T-uptake test [historically called the triiodothyronine resin uptake (T3RU) test] measures the available binding sites on TBG. The test is reported as the thyroid hormone-binding ratio (THBR). The THBR is determined by dividing the percent T-uptake of the patient by that for a normal sample. The ratio is high in hyperthyroidism and low in hypothyroidism. When thyroxine-binding proteins are reduced, the THBR is high; and when binding proteins are elevated, the THBR is low.

The thyroxine-binding globulin (TBG) test measures blood levels of this substance, which is manufactured in the liver. TBG bindsto T3 and T4, and prevents the kidneys from filtering the hormones from the blood. Bound hormone is not physiologically active. The hormone-protein complex is reversible, and in equilibrium with free hormone levels. Therefore, when binding proteins such as TBG are increased, there will be an increase in the amount of total hormone.

Additional tests:

  • Ultrasound exams of the thyroid gland are used to detect signs of growth and other irregularities.
  • Thyroid scans using radioactive iodine or technetium (a radioactive metallic element) reveal the size and activity of the gland. Growths or nodules are seen and can be classified as inactive (cold) or active (hot) depending upon the amount of radioactivity present.
  • Thyroid-specific autoantibodies. Autoimmune disease is the most frequent cause of both hypo- and hyperthyroidism. Commonly performed tests for thyroid autoantibodies are thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), and TSH receptor antibodies (TRAb). Although low levels of these antibodies may be found in healthy persons, elevated levels point to the presence of autoimmune disease that involves the thyroid.
  • Thyroglobulin (Tg) methods are critical for the post-operative management of patients with differentiated thyroid carcinoma (DTC).

Preparation

There is no need to make changes in diet or activities. The patient may be asked to stop taking certain medications until after the test is performed. Venipuncture is performed in the usual manner following standard precautions for prevention of exposure to bloodborne pathogens.

Aftercare

Aftercare consists of routine care of the area around the puncture mark. Pressure is applied for a few seconds, and the wound is covered with a bandage.

Complications

Generally, thyroid function tests are easily interpreted by a physician. However, under certain circumstances interpretation of results is less straightforward. According to an article published in the February 2001 issue of Lancet, one or more of the following features should prompt further investigation:

  • abnormal thyroid function in childhood
  • familial disease
  • thyroid function results inconsistent with the clinical picture
  • an unusual pattern of thyroid function tests results
  • transient changes in thyroid function

Results

Not all laboratories measure all of the thyroid function tests that are available. Different methods may result in different normal ranges. Each laboratory will provide a range of values that are considered normal for each test. Some acceptable ranges are listed below.

KEY TERMS

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.

Graves' disease— The most common form of hyperthyroidism, characterized by bulging eyes, rapid heart rate, and other symptoms.

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.

Hypothyroidism— Underactive thyroid gland; symptoms include fatigue, difficulty swallowing, mood swings, hoarse voice, sensitivity to cold, forgetfulness, and dry/coarse skin and hair.

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).

Reverse T3 (rT3)— An isomer of T3 that is formed from deiodination of T4 in the blood. It is not physiologically active.

Salicylates— Aspirin and certain other nonsteroidal anti-inflammatory drugs (NSAIDs).

T3 Also called triiodothyronine. The more active of the two thyroid hormones.

T4 The principal thyroid hormone, called tetraiodothyronine.

T-uptake test— Also known as the T3 resin uptake test, this test measures the number of available binding sites on TBG.

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.

Thyroid-stimulating hormone (TSH)— A pituitary polypeptide that regulates the activity of the thyroid gland.

Thyrotropin-releasing hormone (TRH)— A neuropeptide produced by the hypothalamus that stimulates pituitary synthesis of TSH.

Thyroxine-binding globulin (TBG)— The primary thyroxine binding protein in blood.

TSH

Normal TSH levels for adults are 0.4-5.0 mU/L.

T4

Normal T4 levels are:

  • 10.1-2.0 microg/dl at birth
  • 7.5-16.5 microg/dl at 1-4 months
  • 5.5-14.5 microg/dl at 4-12 months
  • 5.6-12.6 microg/dl at 1-6 years
  • 4.9-11.7 microg/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

T-Uptake (THBR)

Normal THBR levels are:

  • 0.75-1.05 at birth
  • 0.83-1.15 at 1-15 years
  • 0.85-1.11 for adult males
  • 0.80-1.04 for adult females
  • 0.68-0.87 for second half of pregnancy

LATS

Long-acting thyroid stimulator is found in the blood of only 5% of healthy people.

Health care team roles

Thyroid function tests are ordered and interpreted by a physician. In difficult cases, an endocrine specialist may be needed. A phlebotomist, or sometimes a nurse, collects the blood, and a clinical laboratory scientist CLS(NCA)/medical technologist MT(ASCP) or clinical laboratory technician CLT(NCA)/medical laboratory technician MLT(ASCP) performs the testing.

Resources

BOOKS

Burtis, C.A., and E.R. Ashwood, eds. Tietz Textbook of Clinical Chemistry, 3rd ed. Philadelphia, PA: Saunders, 1999.

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.

PERIODICALS

Boschert, S. "Drugs can alter thyroid function test results." Family Practice News 29, no. 11 (June 1, 1999): 34.

Dayan, C.M. "Interpretation of thyroid function tests." The Lancet 357, no. 9256 (Feb 24, 2001): 619.

Kendall-Taylor, P., et. al. "Thyroid function tests. (Letter to the Editor)." British Medical Journal 321, no. 7268 (Oct 28, 2000): 1080.

O'Reilly, D. "Thyroid function tests time for a reassessment." BMJ 320 (May 13, 2000):1332-1334.

Tate, J., and F. Tasota. "Assessing thyroid function with serum tests." Nursing 31, no. 1 (Jan 2001): 22.

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〉.

OTHER

Spencer, C. "Assay of Thyroid Hormones and Related Substances." 〈http://www.thyroidmanager.org/FunctionTests/assay-frame.htm〉. Revised Aug. 1, 1999.

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Thyroid Function Tests

Thyroid function tests

Definition

Thyroid functions tests are a variety of blood and nuclear medical tests performed to determine if the thyroid is working correctly and to help diagnose the cause if a problem is found.

Purpose

The first purpose of thyroid function tests is to determine if the thyroid is producing the correct amount of hormone. If not, then it is important to detemrine the cuase of the over- or under-production. Thyroid fucntion tests help the doctor determine whenther the cause of the problem is the thyroid itself of if it is the pituitary or a problem with the immune system . Determining the correct cause of the probel mallows the docotr to treat it most effectively.

Precautions

Women who are pregnant or breast feeding should not have a thyroid scan. The very small amount of radioactive substance used has been shown to be safe for adults, but it is not completely clear what effect it might have on a developing fetus. Therefore, women who are pregant should rescheudle the exam for a time after giving birth, or ask their doctor about an alternate imaging test. Women who are breatfeeding should not have thyroid scan because the radioactive material can be passed to the nursing infacnt in the breat milk. If a breastfeeding woman needs a thryodiscan she should make alternate feeding arrangements, such as swithcing to formula, for a few days following the scan until all of the radioactive material has been elimated from her body.

Description

The thyroid is located in the lower part of the neck in the front. It is a gland that is shaped somewhat like a butterfly. It produces thyryd hormones that help the msucles, organs, ans the brain function properly. When the thyroid produces too much horomone it is known ans hyperthyroidism. When the thyroid produces too little hormone it is called hypothyroidism.

The main hormone produced by the thyroid is called thyroxine, also called T4. The reason it is called T4 is because it contains four atoms of iodine. In the body T4 is converted to a hormone called T3 when one of its iodine atoms is removed. Both T4 and T3 levels are important to good health, and separate thyroid funciton tests test for these levels in the blood. Another hormone level improatnt for good heatlh is the level of thryroid stimulating hormone (THS) present in the body. THS is produced by the pituitary gland, and causes the thyroid to relsease T4. THS level can be checked using a blood test.

In some cases when the thryroid appears to not be funcitoning properly it is due to antibodies in the body acting incorrectly. In some people antibodies are produced that attack the thyroid, reducing its hormone output, or stimulate it, increasing its hormone output. Both of these situations can cause serious health consequencse. Antibodies that may be affecitng the thryoid are checked for during a thryroid antibody blood test.

In addition to blood tests, a thyroid scan is sometimes done. This allows the doctor to see an image of the thyroid and how it is working. During a thryroid scan a very small amount of radioactive material, called a radioactive tracer or just a tracer, is either injected into the patinet's arm or swallowed in tablet form. The tracer then enters the bloodstream and ciruclates to the thryoid where it is absorbed.

The tracer gives off a very small amount of gamma radioation. A special camer, called a gamma camera, can detect this radioation, and passes this infroamtion to a computer where an image of the tyroid is produced.

QUESTIONS TO ASK YOUR DOCTOR

  • If the test shows an abnormality, what is the next step?
  • Do I need to stop taking any of my medications before the test?
  • Do I need to eat a special diet before the test?

Preparation

Certain medications can interfere with the Results of the thyroid funciton tests. Which medicaitons may interefere depend on the tests being performed. The paitnet should be sure to tell the doctor all medicaitons that are being taken, inlcyding over-the-counter medications and supplements. The doctor can then determine if any of theses are likely to effect the test Results. If so, the patient may be asked to discontinue taking the medication for one or more days before the test.

Paitnets preparing for a thyroid scan should not eat or drink anything for two hours prior to the test. The doctor may tell the patient not to take certain medications for a few days before the test. The patient may be asked to eat a diet low in iodine before the test. The patient will be given insturcitons specific to his or her case when the test is schedulted

Aftercare

No special Aftercare is required for thyroid function tests.

Complications

Anytime that blood is drawn or an injection is given there is a very small risk of bleeding, swelling, bruising , or infection. There is an extremely small chance that an indiuval may have an allergic reaction to the radioactive tracer used in the thyroid scan.

Results

Normal reslults of thyroid function tests occur when all of the tests show Results within normal ranges for a healthy adult. Abnormla Results can indicacate a variety of different problems. Diagnoses are generally made using the Results from a variety of funciton tsts and diagnositc imaing tests, not on the bases of a single test. The Results of thyroid function tests may indcatie:

  • THS test-Elevated THS levels can indicate primary hypothyroidism, congentical hypothyroidism, thyroid hormone resistance, or tsh-dependent hyperthyroidims. Lowered THS levles can idnicate hyperthyroidism.
  • T3 test-Elevated levels of T3 indicate hyperthyroidism, or rarely thyrotoxicosis or thyroid cancer. Lowered levels of T3 can idnicate hypothyroidism, starvation, or a long-term illness.
  • T4 test-Elevated T4 levels may indicate many thigns incluing Graves disease, Hasimoto's disease, iodineinduced hyperthyroidism, toxic goiter, or chronic thyroidosis. Lowered T4 levels may indicuate hypothyroidism, starvation, or illness.
  • Thyroid anitbody test-Elevated levels in an indivual with hyperthyroidism may indicate autoimmune thyroid disease. Elevated levels in an induval with hypothyroidism may indciate Hashimoto's disease.
  • Thyroid scan-Abnmral scan reslults will be shown as lighter, or darker patches on the images indicating elevated or lowered amounts of tracer absorbtion. Theses reulsts indicate thyroid dysfunction or tumor. The thyroid may appear out of place or mishappen, mich may also indicate a tumor.

Caregiver concerns

A doctor determines the need for thyroid function tests. A nurse trained in drawing blood, a phlebotomist, takes a blood sample from the patient, labels it, and sends it to the laboratory. In the laboraty a laboratory technican performs a varieyt of chemcial and other tests on the blood to determine the level of the homromones of intrest. The Results are then sent to the doctor who ordered the study who communicates them to the patient and determines the need for additional testing or treatment.

When a thryoid scan is performed a nurse injects the patient with the radioactive tracer. A nulcear medicine technician performs the actual scan, contorling the gamma camera and repositioning the patient as necessary during the scan to get all necessary inages. The images from the scan are read by a radiologist or a doctor trained in nuclear meidince. The Results are then sent to the doctor who ordered the test who determines the need for treatment or addional diagnostic tests.

Resources

BOOKS

Fischbach, Frances Talaska, and Marshall Barnet Dunnin III. A Manual of Laboratory and Diagnostic Tests. Philadelphia: Wolters Kluwer Health/Lippincott Willaims & Wilkins, 2009.

Milton, Carl A., ed. Trends in Thyroid Cancer Research. New York: Nova Biomedical Books, 2007.

Rone, James K. The Thyroid Paradox: How to Get the Best Care for Hypothyroidism. Languna Beach, CA: Basic Health Publications, 2007.

PERIODICALS

Cardenas-Ibarra, Lilia, et al. “Cross-Sectional Observartions of Thyroid Function in Geriatric Mexican Outpatients With and Without Dimentia.” Archives of Gerontology and Geriatrics 46.2 (March 2008): 173–181.

Harrison, Pam. “Thyroid Function.” Canadian Living 31.7(July 2006): 57.

ORGANIZATIONS

American Clinical Laboratory Association, 1250 H Street, Suite 880, Washington, DC, 20005, (202) 637-9466, (202) 637-2050, [email protected], www.clinicallabs.org.

Robert Bockstiegel

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"Thyroid Function Tests." The Gale Encyclopedia of Senior Health: A Guide for Seniors and Their Caregivers. . Encyclopedia.com. 20 Sep. 2018 <http://www.encyclopedia.com>.

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"Thyroid Function Tests." The Gale Encyclopedia of Senior Health: A Guide for Seniors and Their Caregivers. . Retrieved September 20, 2018 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/encyclopedias-almanacs-transcripts-and-maps/thyroid-function-tests

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