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Additional Information On Thyroid Testing

Tests to screen for thyroid disease

The two major hormones produced by the thyroid gland are triiodothyronine, or T3, and thyroxine, or T4. Their production is regulated by a feedback system. When blood levels of thyroid hormones decline, the hypothalamus (an organ in the brain) releases thyrotropin releasing hormone, which stimulates the pituitary (a tiny organ below the brain and behind the sinus cavities) to produce and release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce and/or release more thyroid hormones. T4 makes up nearly 90% of thyroid hormones, while T3 makes up less than 10%. Inside the thyroid gland, T4 is bound to a protein called thyroglobulin. When the body requires thyroid hormone, the thyroid gland releases stored T4 into circulation. In the blood, T4 is either free (not bound) or protein-bound (primarily bound to thyroxine-binding globulin). The concentration of free T4 is only about 0.1% of that of total T4. T4 is converted into T3 in the liver or other tissues. T3, like T4, is also highly protein-bound, but it is the free forms of T3 and T4 that are biologically active. Free T3 is also 4 to 5 times more active than free T4 in circulation.

Recommended screening for thyroid disease in vitiligo patients includes tests for anti-TPO antibodies, free T-3, free T-4 and TSH. A description of each of these tests is included below.

Anti-TPO antibodies: Thyroid antibody testing is primarily ordered to help diagnose an autoimmune thyroid disease and to separate it from other forms of thyroiditis. The autoantibodies develop when a person's immune system mistakenly recognizes components of the thyroid as foreign (not-self) and can lead to chronic inflammation of the thyroid (thyroiditis), tissue damage, and disruption of thyroid function. Mild to moderately elevated levels of thyroid antibodies may be found in a variety of thyroid and autoimmune disorders, such as thyroid cancer, Type 1 diabetes, rheumatoid arthritis, pernicious anemia, and autoimmune collagen vascular diseases. Significantly increased concentrations most frequently indicate thyroid autoimmune diseases such as Hashimoto's thyroiditis and Graves' disease. In general, their presence suggests that there is autoimmune thyroid involvement and the higher the level, the more likely that is. Rising levels may be more significant than stable levels as they indicate an increase in autoimmune activity.

Free T-3: used primarily to help diagnose hyperthyroidism and may be ordered to help monitor the progress of a patient with a known thyroid disorder. The T3 test is usually ordered following an abnormal TSH and T4 test. Either the total T3 or the free T3 may be ordered. Since most of the T3 is bound to protein, the total T3 can be affected by protein levels and protein binding ability, but the free T3 is not. Many medications-including estrogen, certain types of birth control pills, and large doses of aspirin-can affect total T3 test results, so tell your doctor about any drugs you are taking. In general, free T3 levels are not affected by these medications.

Free T-4: The total T4 test has been used for many years to help diagnose hyper- and hypothyroidism. It is a useful test but can be affected by the amount of protein available in the blood to bind to the hormone. The free T4 test is a newer test that is not affected by protein levels. Since free T4 is the active form of thyroxine, it is thought by many to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4.

TSH: often the test of choice for evaluating thyroid function and/or symptoms of hyper- or hypothyroidism. Whether high or low, an abnormal TSH indicates an excess or deficiency in the amount of thyroid hormone available to the body, but it does not indicate the reason why. An abnormal TSH test result is usually followed by additional testing to investigate the cause of the increase or decrease.

The following table summarizes test results and their potential meaning.

TSH T4 T3 INTERPRETATION
High Normal Normal Mild (subclinical) hypothyroidism
High Low Low or normal Hypothyroidism
Low Normal Normal Mild (subclinical) hyperthyroidism
Low High or normal High or normal Hyperthyroidism
Low Low or normal Low or normal Nonthyroidal illness; rare pituitary (secondary) hypothyroidism
 
   
 
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