Definition

The FT3 (free triiodothyronine) test is a diagnostic procedure to measure the level of free T3 in the blood. Free T3 is the unbound form of the T3 hormone, meaning it is not attached to proteins and can circulate freely throughout the body.

The T3 hormone is produced by the thyroid, a small butterfly-shaped gland located in the front of the neck. Besides T3, the thyroid also produces another hormone, T4 (thyroxine). Both of these hormones work in tandem to regulate body temperature, maintain weight, and control heart rate.

The FT3 test is particularly useful for diagnosing thyroid diseases, such as hyperthyroidism. Hyperthyroidism occurs when the thyroid overproduces hormones, leading to metabolic disturbances. The FT3 test is usually done alongside T4 and TSH (Thyroid Stimulating Hormone) tests for a more comprehensive evaluation of thyroid health.

Indications

The FT3 test is generally performed under several circumstances:

• To aid in diagnosing thyroid-related disorders.
• To assess thyroid function abnormalities when certain symptoms or signs are present.
• To monitor the progression of a disease after a treatment plan has been initiated.
• To screen individuals at high risk for thyroid disorders, especially those with a family history of thyroid disease.

Contraindications

There are no known contraindications or conditions that would prevent an individual from undergoing this test. It is typically safe for most people to have the FT3 test done.

Preparation Prior to Test

There is no special preparation needed before undergoing the FT3 test, and fasting is not required. However, it is recommended that you consult with your doctor beforehand to ensure that there are no conditions that could interfere with the results of the test, which might not accurately reflect your true health status.

Additionally, inform your doctor about any medications you are currently taking, as some drugs can impact the results of the FT3 test. Medications that may alter the outcomes include:

• Hormonal medications, such as birth control pills
• Thyroid disease treatments
• Steroid medications

Test Procedure

The FT3 test typically requires a blood serum sample of about 0.25 - 0.5 ml. This sample is collected from a vein and placed in a special tube. Laboratory staff will wear gloves and personal protective equipment (PPE) during the sample collection process.

Before taking the blood sample, the technician will apply an elastic band to your arm and clean the area of the needle puncture with antiseptic gauze. The blood is usually drawn from the area inside the elbow. Once the blood is collected in the tube, the elastic band is removed, and the puncture site is pressed and cleaned with antiseptic gauze.

This procedure is quick and typically takes only a few minutes. The blood sample is then analyzed in a laboratory, and the results are usually available within a few hours.

Normal and Abnormal Values

The normal range for FT3 levels can vary by age. Below is a general guide to the normal FT3 values:

As shown in the table, FT3 levels in the blood have normal ranges that vary based on age. If your results fall outside the normal range, it may indicate an issue with your thyroid function.

It is important to note that each laboratory may have slight differences in the reference values due to variations in equipment and methods used.

Results and Recommendations (Follow-up Tests)

Low

If your FT3 test results show low levels, you might be experiencing hypothyroidism. However, the FT3 test should be followed by additional tests, such as FT4 and TSH, to gain a clearer understanding of the underlying condition.

Several conditions can lead to low FT3 levels, including thyroiditis, malnutrition, and chronic illnesses.

Even if your FT3 levels are low, it’s essential not to self-diagnose. You should consult with your doctor for a full consultation and physical examination, particularly if you're experiencing any concerning symptoms.

Normal 

If your FT3 test results are normal, it suggests you likely have no thyroid-related issues, provided you aren’t showing any symptoms or signs of thyroid disease.

To continue preventing any potential thyroid problems, it’s important to maintain a healthy lifestyle. This can help keep your FT3 levels within the healthy range and avoid any imbalances in the future.

High

If your FT3 test results indicate elevated levels, you should consult your doctor, as this may suggest hyperthyroidism.

Several conditions can result in high FT3 levels, such as thyrotoxicosis, toxic nodular goiter, and liver disease. Additionally, medications like thyroid hormone treatments, birth control pills, and hormones such as estrogen can also lead to increased FT3 levels.

Consult the Right Doctor

If you have abnormal FT3 test results, you should consult with a general practitioner for further evaluation and to begin the appropriate treatment plan. If necessary, you can also consult with a specialist in internal medicine, metabolic disorders, or endocrinology to get a more comprehensive understanding of your FT3 levels and thyroid function. For pediatric patients, if the FT3 test results are abnormal, it is important to seek advice from a pediatrician who can provide a tailored approach to diagnosis and treatment.

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