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Thyroid function

Why

To help assess thyroid gland function and to help diagnose thyroid disorders.

When to be tested?
When you have signs and symptoms that suggest hypo- or hyperthyroidism due to a condition affecting the thyroid.

Champion required?
A blood sample drawn from a vein in the arm.

Do you need test preparation?

None required; however, some medications may interfere with the tests included in the panel, so tell your healthcare provider about any medications you are taking.

A thyroid panel is a set of tests that can be ordered together to help assess thyroid gland function and to help diagnose thyroid disorders. Tests included in a thyroid panel measure the amount of thyroid hormones in the blood. These hormones are chemicals that travel through the bloodstream and control or regulate the body’s metabolism – how energy works and uses.

The thyroid panel usually includes:

  • TSH (thyroid stimulating hormone) – to test for hypothyroidism, hyperthyroidism and to monitor the treatment of a thyroid disorder.
  • T4 free or T4 (thyroxine) – to test for hypothyroidism and hyperthyroidism; can also be used to monitor treatment.
  • T3 free or total T3 (triiodothyronine) – to test for hyperthyroidism; can also be used to monitor treatment.

Sometimes a T3 resin retraction test (T3RU) is included to calculate, along with the T4 value, the free thyroxine index (FTI), another method for assessing thyroid function that corrects changes in certain proteins that may affect total T4 levels.

TSH is produced by the pituitary gland and is part of the body’s response system to maintain stable amounts of thyroid hormones T4 and T3 in the blood. When thyroid hormone levels drop, the pituitary gland is stimulated to release TSH. TSH, in turn, stimulates the production and release of T4 and T3 by the thyroid gland. When the system is functioning normally, thyroid production switches on and off to maintain a constant level of thyroid hormones in the blood.

T3 and T4 are the two main hormones produced by the thyroid gland, a small butterfly-shaped organ. Together they help control the degree to which the body uses energy. Almost all T3 and T4 circulating in the blood are protein bound. Small parts that are not bound or “free” are biologically active forms of hormones. Tests can measure the amount of free T3 or free T4 or total T3 or total T4 (forced plus unlimited) in the blood.

Total T4 and total T3 tests have been used for many years, but they can be affected by the amount of protein available in the blood to bind to the hormone. FreeT4 and free T3 tests are not affected by protein levels and are thought by many to be more accurate reflections of thyroid hormone function. In most cases, the free T4 test has replaced that of the total T4 test. However, some professional guidelines recommend a total T3 test, so either a total T3 test or a free T3 test can be used to assess thyroid function.