Thyroid function tests

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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).

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.

— Maureen Haggerty

Tests for thyroid function (for example, radioactive iodine uptake, protein-bound iodine, basal metabolic rate, serum cholesterol, triiodothyronine suppression, thyroid-stimulating hormone 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.

(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.

— MARTIN I. SURKS

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