Free Thyroxine


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Free T4 is the metabolically active fraction of thyroxine and is the precursor of triiodothyronine (T3). Free T4 is a sensitive test for thyroid function and is increased with hyperthyroidism and decreased with hypothyroidism. Free T4 is not affected by changes in thryoxine-binding proteins. Information from Laboratory Test Handbook, Jacobs DS, DeMott WR, and Oxley DK, LexiComp, 2001.

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Purpose of the test

The Thyroxine (T4) Test is a blood test designed to diagnose thyroid conditions. The thyroid, a small gland located at the base of the throat, produces hormones that regulate various bodily functions, including metabolism, temperature, and mood. The T4 test specifically measures the level of thyroxine, a type of thyroid hormone, in the blood. Abnormal levels of T4 can indicate thyroid disease.

Who Should Get Tested

Individuals suspected of having a thyroid problem may require a T4 test. Typically, a TSH test is conducted first, and if the results are abnormal, a T4 test follows. Symptoms of thyroid imbalances, such as weight gain, fatigue, sensitivity to temperatures, and irregular menstrual periods, can also prompt this test.

Preparing for the Test
Depending on the type of cholesterol test, you may be required to fast for 9 to 12 hours before the test. This means avoiding all food and drinks except water. Fasting is usually required for a lipid panel, which measures LDL and HDL cholesterol, and triglycerides.
Inform your healthcare provider about any medications you are currently taking, including over-the-counter medications and supplements, as they can affect your test results.
Avoid alcohol consumption for at least 24 hours before the test, as alcohol can raise triglyceride levels.
Avoid heavy exercise for 24 hours before the test, as this can also affect cholesterol and triglyceride levels.
Avoid smoking or using any nicotine products for at least 30 minutes before the test, as they can temporarily increase cholesterol levels.
Follow instructions
Follow any specific instructions provided by your healthcare provider, such as the time of day to take the test or any dietary restrictions.
Interpreting Test Results

T4 test results can be reported as "free T4" or "free T4 index (FTI)." Lower than normal T4 levels might indicate conditions like hypothyroidism or thyroiditis, while higher levels can suggest hyperthyroidism, thyroiditis, or other conditions. It's crucial to consult with a healthcare provider to interpret the results in the context of individual health and symptoms.

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