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.

Blood Test Preparation Guidelines
Time of day
It is recommended to schedule your appointment for blood tests in the morning hours between 7:00-10:00.
It is recommended to fast for approximately 12 hours before blood sampling. The last meal of the previous day should ideally be consumed around 6:00 p.m. On the day before the test, avoid heavy and fatty meals as well as alcohol.
Stay hydrated
Drinking water prior to testing can help with sample collection.
Other factors
Tests should not be performed after a sleepless night or intense physical activity. It is recommended to avoid exercise and stress immediately before blood collection and to not smoke. A short rest is recommended.
Medications and supplements:
Blood samples should be collected before the morning dose. Some drugs can interfere with test results. Consult with your doctor whether you can delay your dose because of lab tests.
Biotin supplements:
High doses of biotin supplements can affect test results, causing false elevation or reduction. It's recommended to avoid taking biotin for at least 72 hours before blood collection. If you are taking biotin, inform the personnel collecting the blood so that they can provide specific instructions.
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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