Thyroid function tests

Thyroid function tests are blood tests used to evaluate how effectively the thyroid gland is working. These tests include the thyroid-stimulating hormonetest (TSH), the thyroxine test (T4), the triiodothyronine test (T3), the thyroxine-binding globulin test (TBG), the triiodothyronineresin uptake test (T3RU), and the long-acting thyroid stimulatortest (LATS).

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 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, anabolicsteroids, phenytoin, and thyroid medications may be discontinued prior to athyroxine-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, andthyroid medications before having a triiodothyronine resin uptake (T3RU) test. Prior to taking a long-acting thyroid stimulant (LATS) test, thepatient will probably be told to stop taking all drugs that could affect test results.

Most doctors consider the sensitive thyroid-stimulating hormone (TSH) test tobe the most accurate measure of thyroid activity. By measuring the level ofTSH, 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, standardblood tests measured levels of T4 and T3 to determineif 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 manufacturedin 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 itis 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 thethyroid to produce and secrete abnormally high amounts of hormones.

It takes only minutes for a nurse or medical technician to collect the bloodneeded 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 laboratoryfor testing. The patient will usually feel minor discomfort from the "stick"of the needle. Not all laboratories measure or record thyroid hormone levelsthe same way. Each laboratory will provide a range of values that are considered normal for each test.

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