What is FT3?
A free triiodothyronine test, most commonly referred to as a T3 test, measures the level of a hormone called triiodothyronine in your blood. T3 is a hormone secreted by the thyroid glands and is important for the metabolism of your body including the heart rate and temperature. A T3 test is required in case of symptoms like anxiety, weight loss, low tolerance for heat, and trouble sleeping. This test is most often used to diagnose hyperthyroidism, a condition in which the body makes too much thyroid hormone.
T3 test is performed with T4 and TSH tests to get a full picture of how your thyroid is functioning. Overnight fasting of 10-12 hours is advised before undergoing free triiodothyronine test.
Why is FT3 done?
To diagnose thyroid diseases (Hypothyroidism or Hyperthyroidism)
To monitor the patient’s response to thyroid diseases’ treatment
To evaluate thyroid status in patients with altered distribution of thyroid binding proteins (e.g. pregnancy, dysalbuminemia)
What does FT3 Measure?
The Free Triiodothyronine (T3) Test measures the levels of the Free form of the T3 hormone.
The thyroid gland secretes the following hormones:
Thyroid Stimulating Hormone (TSH), also called Thyrotropin is a hormone secreted into the blood by the Pituitary gland (a gland present in the brain). It directs the thyroid gland to produce and release the thyroid hormones (T3 & T4) into your blood. The iodine absorbed from the food stimulates the thyroid glands to make the thyroid hormones.
The thyroid hormones are essential for growth and metabolism. If the thyroid gland produces very high amounts of T3 & T4 hormones, you may experience symptoms of weight loss, rapid heartbeat, tremors, sweating, anxiety, increased sensitivity towards heat, etc. and this is known as Hyperthyroidism.
The decreased production of thyroid hormones results in Hypothyroidism which may cause weight gain, fatigue, slow heart rate, increased sensitivity towards cold, depression, dry and thin hair, etc.
There is a feedback system in the body to maintain stable amounts of the thyroid hormones (T3 and T4) in the blood. When the levels of thyroid hormone decrease, the pituitary gland is stimulated to release TSH. This high TSH, in turn, leads to an increase in the release of thyroid hormones (T3 & T4) from the thyroid gland and vice-versa.
T3 hormone circulates in the blood in two forms:
1) Bound form - It is bound to the proteins present in the blood which prevents it from entering the body tissues.
2) Free form - It enters the body tissues where it is needed and thus is the active form.
The total T3 includes both the bound and the free forms circulating within the blood and can be affected by the amount of protein available in the blood to bind to them.
Majority of the T3 hormone is formed from T4 hormone and a smaller fraction is produced directly by the thyroid gland. Free Triiodothyronine (FT3) constitutes of only 0.3% of the total T3 hormone. The two main proteins in the blood that the T3 hormone binds itself to are albumin and Thyroxine-binding globulin (TBG), also called Thyroid hormone Binding Globulin (THBG).
Hence, the T3 hormone can be measured as Free T3 or Total T3. Free Triiodothyronine (T3) Test is also a part of the Thyroid profile Free test which includes two more tests: Thyroid Stimulating Hormone (TSH) and Free Thyroxine (FT4).
Interpreting FT3 results
The results show either excessively high or low levels of free T3 in the blood samples. The normal range for free T3 is from 1.8 to 5.4 mcg/dl for men and women of all age groups.