The major thyroid hormones, T4 and T3 can be directly measured in the blood as can the pituitary hormone called TSH (Thyroid Stimulating Hormone) that reflects the levels of thyroid hormones. Blood testing is highly accurate and is the most common form of testing to diagnose thyroid disorders.
Thyroxine-T4
This metabolism-regulating hormone manufactured by the thyroid gland is the most abundant in the body but is less powerful than the T3 hormone (to be addressed in a later subheading). The ratio of T4 compared to T3 is approximately 75% T4 to 25% T3. Thyroxine-T4 is a reserve hormone that remains on standby (precursor) to be converted into Triiodothyronine-T3, the hormone most responsible for metabolism-regulation as needed in the body. Any surplus of the T4 that is not used-up during the conversion process is instead converted into a substance called Reverse-T3, which means it has been rendered inactive, so that any excess does not remain in the body and cause an overactive metabolism or “hyperthyroidism”.
T4 Blood Tests
Blood tests that measure the T4 come in a variety of versions, including the “Total T4” (also called Thyroxine), the “Free T4” (considered the best by some specialists) and the “Free Thyroxine Index” (FTI). All of the tests that help determine the T4 level in the body have lab ranges (normal values) with a flagged-high reading indicating hyperthyroidism and a flagged-low reading indicating hypothyroidism. If for example the T4 has a reference range at a testing lab of “4.0 to 12.0”, a reading above 12.0 is suggestive of an overactive thyroid gland. A reading below 4.0 would be suggestive of an under-active thyroid gland. High-normal or low-normal readings and those that are borderline, are also suspect for developing thyroid hormone disorders and merit follow up retesting. Thyroid doctors who treat thyroid hormone imbalances use a narrowed therapy treatment goal that would keep the T4 level at approximately between mid range and higher normal.
Triiodothyronine-T3
This thyroid hormone comes directly from the T4 via the conversion process previously mentioned, unless administered orally from an outside source. The T4 accomplishes this by dropping one iodine atom from its molecules which transforms it into the more metabolically active T3. Medical sources state that the manufactured T3 is about five times more powerful than T4 in regulating the metabolism. In certain types of hypothyroidism, the T3 will go low while the T4 level remains normal. There are a variety of names for this type hypothyroidism but all can be referred to under the term “Low T3 Syndrome”. The condition is often diagnosed by measuring the blood-level of Reverse T3 in the body, which becomes highly elevated when there is a normal amount of T4 that is not being converted into enough T3 to support metabolism.
T3 Blood Tests
This hormone is also ordered in either the “Total” or “Free” level and just like the T4, a result that is elevated above normal, points to a diagnosis of hyperthyroidism, while a below normal result points to hypothyroidism. T3 is usually tested in combination with T4 and TSH and is rarely tested alone unless a patient with Low T3 Syndrome is being treated with T3-only replacement hormone therapy.
The normal range for Total T3 is approximately “80 to 225” and the normal range for Free T3 is approximately “2.0 to 4.0”. Above normal readings help to diagnose cases of hyperthyroidism and below normal readings help to diagnoses hypothyroidism.
T3 Resin Uptake Test
One blood test that is sometimes included on thyroid panels is the “T3 Resin Uptake”. This test should not be confused with those that measure T3 levels in the body. This test instead, measures how well the blood uptakes the T3 hormone. A patient’s blood sample has T3 added to it at the lab and if uptake of the hormone into the blood for use in the cells and tissues of the body, is hindered (not adequate), this helps to determine a disease process that is blocking this action.
Autoimmune thyroid diseases that present with auto-antibodies that attack thyroid proteins can be a cause of a low T3 Resin Uptake. One type of thyroid antibody that can hinder T3 from up-taking in the blood, is the one that attacks “thyroxine-binding globulin” (TBG) called “anti-TBG antibodies”. TBG is the protein that binds with thyroxine in the blood stream, so that it can enter receptor-cells and tissues and convert into T3 as needed. High or low levels of T3 Uptake can mean different things depending upon how it compares with thyroid hormone levels and the level of TBG itself.
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