Thyroid Function Tests Health Article

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 triiodothyro-nine 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 triiodothyro-nine (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

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