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), 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.

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.

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.