How can we help?

Home

FT3 & FT4 (Free T3 & Free T4)

Also referred as

Free T3 & Free T4

3,064+ booked recently

For men & women

Earliest reports in

18 hours

Contains

2 tests

Know more about this test

An FT3 & FT4 (Free T3 & Free T4) test measures the amount of free form of triiodothyronine (FT3) and thyroxine (FT4) hormones in the blood. This test helps evaluate the functioning of the thyroid gland and detect possible thyroid disorders, including hyperthyroidism and hypothyroidism.

Samples required

Blood

Find out

Why is this test booked?

Preparation for this test

Sample Collection

Who will collect your samples?

Understanding FT3 & FT4 (Free T3 & Free T4)

The thyroid gland is a butterfly-shaped organ in the neck that produces hormones like thyroxine (T4) and triiodothyronine (T3), which are responsible for regulating metabolism and other bodily functions. The production and release of these hormones are controlled by the pituitary gland's thyroid-stimulating hormone (TSH). FT4 and FT3 represent the free or unbound, active forms of thyroxine and triiodothyronine. It reflects the amount of available thyroid hormone circulating in the blood.

An FT3 & FT4 (Free T3 & Free T4) test is used to assess how well your thyroid gland is working and aid in the diagnosis and management of thyroid disorders, such as an overactive thyroid gland (hyperthyroidism) or an underactive thyroid gland (hypothyroidism). Your doctor may advise this test if you show symptoms suggestive of hyperthyroidism, such as weight loss, nervousness, irritability, increased sweating, and rapid heartbeat, or hypothyroidism, such as weight gain, fatigue, sensitivity to cold, constipation, and dry skin. It is also important for pregnant women to have healthy amounts of thyroid hormones to ensure the healthy development of their babies.

No special preparations are needed for this test. Usually, you do not need to stop taking your thyroid medication before taking this test. However, let your doctor know about all your medications, as some can affect thyroid function. Biotin (Vitamin B7) is a commonly taken over-the-counter supplement that can lead to some aberrancy in your test results even when the values are normal. Therefore, avoid taking biotin two days before an FT3 & FT4 test.

Test result ranges are approximate and may differ slightly between labs depending on the methodology and laboratory guidelines. Talk to your doctor about your specific results. The test results will help them determine your medical condition, recommend lifestyle modifications such as diet and exercise, evaluate whether or not medication will be required to manage your condition and formulate an overall treatment plan.

What does FT3 & FT4 (Free T3 & Free T4) measure?

Contains 2 tests

An FT3 & FT4 (Free T3 & Free T4) test measures the free form of triiodothyronine (FT3) and thyroxine (FT4) hormones made by the thyroid gland. These hormones exist in the blood in two forms: bound (attached to proteins) and free (not attached to proteins). Most of the T3 and T4 hormones circulating in the blood are bound to proteins and only a small part circulates freely and constitutes the physiologically active form. The body maintains a delicate balance of bound and free T3 as well as T4 to ensure proper functioning. Measuring free forms of thyroid hormones (FT3 and FT4) gives a better understanding of thyroid function than the bound forms as they are independent of changes in the concentration and the binding properties of the thyroid hormone binding proteins.

Thyroxine - Free (FT4)

The Thyroxine - Free (FT4) test measures the free form of thyroxine (T4) hormone made by the thyroid gland. T4 hormone exists in the blood in two forms: bound (attached to proteins) and free (not attached to proteins). Most of the T4 hormone circulating in the blood is bound to proteins and only a small part is free. A delicate balance of bound and free T4 should be maintained to ensure the proper functioning of the body.

Know more about Thyroxine - Free (FT4)

Triiodothyronine - Free (FT3)

The Triiodothyronine - Free (FT3) test measures the levels of free triiodothyronine (T3), a hormone produced by the thyroid gland. T3 hormone plays a vital role in regulating the body's metabolism, energy levels, and growth & development. It exists in the blood in two forms: free T3 and bound T3. Only a small portion of T3 is free (not bound to proteins) and can be utilized by the body’s tissues. In contrast, more than 99% of T3 is bound to proteins, such as albumin and thyroid-binding globulin (TBG), preventing it from entering the tissues. The body balances bound and free T3 to ensure proper functioning.

Know more about Triiodothyronine - Free (FT3)

Frequently booked together

FAQs related to FT3 & FT4 (Free T3 & Free T4)

Why is the Free T3 & Free T4 test done?

The Free T3 & Free T4 test is done to check for hyperthyroidism or hypothyroidism when you show symptoms like weight gain/loss, fatigue, dry skin, hair thinning, slow heart rate, muscle weakness, anxiety, trouble sleeping, or frequent bowel movement.

Can I take the Free T3 & Free T4 test during pregnancy?

Yes, you can take the Free T3 & Free T4 test during pregnancy. This test is usually recommended along with some other tests during pregnancy to measure the levels of thyroid hormones as their deranged levels can complicate pregnancy.

Is fasting required for the Free T3 & Free T4 test?

No, fasting is not needed before taking this test. However, your doctor will guide you if any special preparation is required.

What happens when T3 and T4 levels are high?

High T3 and T4 levels can lead to a condition called hyperthyroidism in which the thyroid gland produces too much thyroid hormones. Some of the common causes of hyperthyroidism include nodular goiter and thyroiditis.

What happens when T3 and T4 levels are low?

Low T3 and T4 levels can lead to a condition called hypothyroidism in which the thyroid gland produces less T3 & T4 hormones. Some of the common symptoms of hypothyroidism are fatigue and weakness, weight gain, cold intolerance, and constipation.

What factors can affect the results of the Free T3 & Free T4 test?

Certain medications like methadone, birth control pills, or steroids can affect your free T3 levels. Similarly, phenobarbital, amiodarone, or diclofenac can have an impact on your free T4 levels. Besides this, severe cases of chronic ailments, such as liver cirrhosis or chronic kidney failure, can also affect your Free T3 and Free T4 test results.

Content created by

Dr. Swati Gupta (Anand)

MD Lab Medicine, MBBS

Want to know moreRead our editorial policy

Last Updated

9th Oct 2025