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The Test Sample
What is being tested?
Thyroxine (T4) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called triiodothyronine (T3) and together they help control the rate at which the body uses energy. Almost all of the T4 (and T3) found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. This test measures the amount of free T4 in the blood.
T4 production is regulated by a feedback system. When the level of T4 in the bloodstream decreases, the hypothalamus releases thyrotropin releasing hormone, which stimulates the pituitary gland to produce and release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to make and/or release more T4. As the blood concentration of T4 increases, TSH release is inhibited.
T4 makes up about 90% of thyroid hormones. When the body requires thyroid hormone, the thyroid gland releases stored T4 into circulation. In the blood, T4 is either free (not bound) or bound to protein (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 mostly bound to protein, but it is the free forms of T3 and T4 that are biologically active. Free T3 is 4 to 5 times more active than free T4 in circulation.
If the thyroid gland does not produce sufficient T4, due to thyroid dysfunction or to insufficient TSH, then the affected person experiences symptoms of hypothyroidism such as weight gain, dry skin, cold intolerance, irregular menstruation, and fatigue. Severe untreated hypothyroidism, called myxedema, can lead to heart failure, seizures, and coma. In children, hypothyroidism can stunt growth and delay sexual development.
If the thyroid gland produces too much T4, the rate of the person's body functions will increase and cause symptoms associated with hyperthyroidism such as increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, and puffiness around dry, irritated eyes.
The most common causes of thyroid dysfunction are related to autoimmune disorders. Graves disease causes hyperthyroidism and Hashimoto thyroiditis causes hypothyroidism. Both hyperthyroidism and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and excessive or deficient production of TSH. The effect of these conditions on thyroid hormone production can be detected and monitored by measuring the free T4.
How is the sample collected for testing?
A blood sample is obtained from a needle placed in a vein in the arm or from pricking the heel of an infant.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed. Certain medications can interfere with the free T4 test, so tell your health practitioner about any drugs that you are taking. If you take thyroid hormone as treatment for thyroid disease, it is recommended that your blood sample be drawn before you take your dose for that day.