thyroid function tests


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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:
  • 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

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〉.

thyroid function tests

Tests of the production of the thyroid hormones thyroxine (T4), which has four iodine atoms in the molecule, and triiodothyronine (T3), which has three. This can be done by measuring the rate at which radioactive iodine is accumulated by the gland. The amounts of the two hormones in the blood can also be measured in the laboratory using samples of serum taken from the patient. Thyroid function can be measured indirectly by assessing the rate of the body's oxygen consumption in the resting state. This is called the basal metabolic rate (BMR).

thyroid

1. resembling a shield.
2. the thyroid gland (see below) secreting thyroid hormones (see below).
3. a pharmaceutical preparation of cleaned, dried, powdered thyroid gland, obtained from those domesticated animals used for food by humans.

accessory thyroid
an additional thyroid located anywhere from the larynx to diaphragm, e.g. intrapericordial aorta; may be sufficient to supply the patient's need of thyroid hormone. Most common in dogs.
thyroid C cell
see c cell.
thyroid cartilage
the shield-shaped cartilage of the larynx.
thyroid diverticulum
primordium of the thyroid gland; appears as an outgrowth of the foregut between the first two pharyngeal pouches. This tube of epithelial cells grows ventrally into mesenchyme; the tube becomes the thyroglossal duct, the tip divides into two lobes, the thyroid glands.
thyroid dysfunction
ectopic thyroid
migration of thyroid diverticulum tissue to aberrant sites occurs, e.g. thyroid tissue in the thymus. These cause no apparent abnormality.
thyroid extract
a pharmaceutical substance derived from thyroid glands, used in the treatment of hypothyroidism. See thyroid (3) (above).
thyroid function tests
used to assess the functional capacity of the thyroid glands; most commonly employed in dogs and cats. Include plasma T4, plasma T3, free T4 radioiodine uptake, and TSH response tests.
thyroid gland
the largest of the endocrine glands, situated in the neck caudal to the larynx. It produces hormones (see below) that are vital in maintaining normal growth and metabolism. It also serves as a storehouse for iodine.
Enlarge picture
Microscopic appearance of the thyroid gland. By permission from Guyton R, Hall JE, Textbook of Medical Physiology, Saunders, 2000
thyroid hormones
iodothyronines secreted by the thyroid gland, principally thyroxine (tetraiodothyronine, T4) and tri-iodothyronine (T3); derived from iodination of tyrosyl residues in thyroglobulin. The pharmaceutical names for T4 and T3 are levothyroxine and liothyronine, respectively. Regulate basal metabolic rate.
thyroid parafollicular cell
see c cell.
thyroid radioiodine uptake
used as a thyroid function test but superseded these days by estimates of T4 (thyroxine).
thyroid-stimulating hormone (TSH)
thyroid tumor
mostly follicular adenomas in old-aged dogs, cats and horses; papillary adenomas are rare.
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