Thyroid test for evaluating thyroid function.
As long as low thyroid hasnt been recognized as a disease, the tests for it havent ever been equal to the task of diagnosis. Modern medicine has been in search of accurate thyroid function tests. The following is the history of testing thyroid function
Fifty years ago there was laborious time consuming test called the basal metabolic rate (BMR). This breathing test was only partially accurate in differentiating who had low thyroid and who didnt. This test required the patient to breathe through a tube for several hours while lying completely still. The purpose was to measure the amount of calories required to sustain the bodys metabolic processes at rest.
Protein bound iodine (PBI) soon replaced BMR. The PBI test provided a rough estimate of the amount of T-3 and T-4 circulating in the bloodstream. This test measured the amount of iodine in the body that was bonded to protein carrier molecules.
Eventually, scientists learned how to measure thyroxine (T-4) directly from a blood sample. However, this didnt last because of the realization that thyroxine wasnt the active thyroid hormone in the body. The active hormone is T-3. T-4 must first be converted to T-3.
Doctors became able to measure the conversion of T-4 to T-3 indirectly by a test called the T-3U. The T-4 and T-3U was calculated together to determine a new number called the Free Thyroxine Index (FTI). FTI is the estimated amount of biologically active thyroid hormone in circulation. T-4, T-3U, and FTI became known as the thyroid panel. These days, the thyroid panel is considered by many doctors to be inaccurate.
The thyroid panel isnt as sensitive and specific a test for thyroid stimulating hormone (TSH) secreted from the pituitary gland. The amount of TSH in the blood is a useful indicator of whether ones thyroid gland is functioning either too high or too low. Since the TSH test has the ability to test for both high and low thyroid, this test has been utilized by itself as the only thyroid test ordered for screening purposes. However, this isnt the only test needed.
Total T-3 also called total triiodothyronine is a direct measurement of thyronine. This test can be helpful because in some patients that test high or low thyroid, this test showed some abnormalities in patients that tested normal in the thyroid panel and TSH.
Free T-4 and Free T-3
Almost all thyroid hormone in the bloodstream is tightly bound to blood carrier protein. In that form, it isnt available to enter cells. Free T-3 and Free T-4 are helpful in determining who has low thyroid when other tests are normal.
Thyrotropin releasing hormone test (TRH) is secreted by the hypothalamus. TRH is the most sophisticated of all tests to reveal mild low thyroid. The TRH test is different than the TSH test in that, the TSH test only measures the amount of thyroid stimulating hormone normally being released from the pituitary. The TRH measures how much TSH the pituitary keeps in reserve. As of yet, you can see there are no set of thyroid tests that can absolutely rule in or rule out the disease in all of the people tested for it.
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