Free Thyroxine T4
The thyroid gland produces and secretes T4, otherwise known as thyroxine. Proteins bind to T4 and carry it throughout the bloodstream. Once in the tissues, T4 is released from the proteins and is now free to convert into the more active form called T3. For this reason, many doctors believe that measuring free T4 is a more sensitive test for thyroid hormone production.
What is Free T4?
The thyroid gland produces and secretes T4, otherwise known as thyroxine. Proteins bind to T4 and carry it throughout the bloodstream. Once in the tissues, T4 is released from the proteins and is free to convert into the more active form called T3. For this reason, many doctors believe that measuring free T4 is a more sensitive test for thyroid hormone production.
The free T4 (FT4) assay measures the concentration of free thyroxine, the only biologically active fraction, in the serum (about 0.05% of the total T4). The free thyroxine is not affected by changes in concentrations of binding proteins such as TBG and thyroid-binding prealbumin. Thus such conditions as pregnancy or estrogen and androgen therapy do not affect the FT4. Therefore the FT4 assays generally are considered to provide a more reliable indication of actual thyroid status because only the free hormone is physiologically active. In developing hypothyroidism, T4 (free T4) is the more sensitive indicator of developing the disease than T3 (Free T3) and is therefore preferred for confirming hypothyroidism that a high TSH result has already suggested.
· The thyroid gland produces and secretes T4, otherwise known as thyroxine. Proteins bind to T4 and carry it throughout the bloodstream.
· In the tissues, T4 is released from the proteins and is free to convert into the more active form called T3.
· Many physicians believe that measuring free T4 is a more sensitive test for thyroid hormone production
High Free T4
High-free T4 is associated with anxiety, insomnia, diarrhea, and muscle aches and pains. These symptoms collectively are known as hyperthyroidism.
Low Free T4
Low levels of free T4, known as hypothyroidism, are associated with symptoms such as depression, weight gain, dry skin, hair loss, constipation, and cold intolerance.
Free T4 Range
· Reference Interval: 0.82−1.77 ng/dl (>19 yr old)
· Methodology: Electrochemiluminescence Immunoassay (ECLIA)
Fasting is not required. Take all medications as prescribed. Do not take supplements on the day of the test. This test is also included in the Comprehensive Thyroid Profile.
NOTE: If you are taking biotin, please stop it 72 hours before the blood draw since it may interfere with hormone blood tests.
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Go to the "Find a Location" page to find the closest lab location. No need to make an appointment since walk-ins are welcomed. Once you have identified your closest location, go to step 2.
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Go to "Choose a Test" and add your selection (s) in the shopping cart. If you prefer to save money on bundled tests, we have created "Lab Test Panels" that can help you decide what to order.
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Pay using a credit card.
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