Free T3


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Free T3 (free triiodothyronine) is a measure of free circulating T3, excluding that which is bound to carrier proteins. This provides a more accurate measure of T3 levels because the carrier proteins can increase or decrease in various conditions artificially inflating or shrinking the total T3. T3 is discriminating for diagnosing and/or following the treatment of hyperthyroidism. It is not very useful at the low range of concentrations, so is not very helpful in diagnosing hypothyroidism. The thyroid delivers 20 times more T4 than T3, so T4 is the more relevant analyte except in special cases.

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Purpose of the test

The Triiodothyronine (T3) test measures the level of T3 in the blood. T3 is one of the two primary hormones produced by the thyroid gland, with the other being thyroxine (T4). These hormones play a pivotal role in regulating the body's energy usage, controlling weight, body temperature, muscle strength, and the nervous system. The T3 hormone exists in two forms: bound T3 (attaches to protein) and free T3 (does not attach). The test can measure both forms (total T3 test) or just the free T3. Abnormal T3 levels can indicate thyroid disease.

Who Should Get Tested

A T3 test is primarily used to diagnose hyperthyroidism, where the body produces excessive thyroid hormone. The test is often ordered alongside T4 and TSH tests. It may also be used to monitor treatment for thyroid disease. Individuals showing symptoms of hyperthyroidism, such as anxiety, weight loss, tremors, increased heart rate, bulging eyes, sleep disturbances, fatigue, heat intolerance, and frequent bowel movements, might need a T3 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

High total or free T3 levels may indicate hyperthyroidism, while low levels might suggest hypothyroidism. T3 test results are often analyzed in conjunction with T4 and TSH test results for a comprehensive diagnosis of thyroid disease. If there are concerns about the results, it's crucial to discuss them with a healthcare provider. During pregnancy, thyroid changes can occur, but T3 testing is usually not required unless specific conditions or symptoms are present.

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