Free Triiodothyronine (fT3) – DM6 Health


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Free Triiodothyronine (fT3)


What is free triiodothyronine (fT3) and what is it used for in the body?

Free triiodothyronine (fT3) is a hormone produced by the thyroid gland that plays a key role in regulating metabolism. It is one of two main hormones produced by the thyroid gland, along with thyroxine (T4). fT3 is more biologically active than T4 and is responsible for the majority of the effects of thyroid hormone on the body's tissues.

How is fT3 measured in the body?

fT3 can be measured through a blood test. The test measures the amount of fT3 in the blood, and the results are typically reported in picograms per milliliter (pg/mL).

What are normal levels of fT3?

Normal levels of fT3 can vary depending on the laboratory and the specific test used, but a general range is 2.3 to 4.2 pg/mL. It is important to note that normal values may also vary based on factors such as age, sex, and overall health.

What can cause abnormal levels of fT3?

Abnormal levels of fT3 can be caused by several factors, including thyroid disorders such as hyperthyroidism or hypothyroidism, liver disease, and certain medications.

How is fT3 used in the diagnosis and management of thyroid disorders?

fT3 is typically measured along with thyroxine (T4) and thyroid-stimulating hormone (TSH) in the evaluation of thyroid function. Abnormal levels of fT3 can be an indication of an underlying thyroid disorder, and the specific pattern of fT3, T4, and TSH levels can help guide the diagnosis and management of these conditions.

Can fT3 levels be affected by other factors such as stress or diet?

Yes, fT3 levels can be affected by a variety of factors, including stress, diet, and certain medications. It is important to discuss any changes in these factors with a healthcare provider, as they may need to be taken into account when interpreting fT3 levels.

Are there any potential risks or side effects associated with fT3 testing?

fT3 testing is generally considered safe and carries a low risk of side effects. The most common side effect is minor discomfort or pain at the site of the blood draw.


Mayo Clinic. (2020). Triiodothyronine (T3) blood test.

Lab Tests Online. (2020). Free triiodothyronine (fT3) test.

American Thyroid Association. (2021). Laboratory evaluation of thyroid function.

EndocrineWeb. (n.d.). Thyroid hormones T3 and T4: What you need to know.