Free Thyroxine (FT4) Test

Free Thyroxine (Ft4) Test

It is a simple blood test which measures free thyroxine (Ft4) levels in blood. This is one of the tests used to assess function of thyroid gland.

Thyroid gland is a tiny butterfly shaped gland present near the throat which secretes thyroid hormones in the body. This gland is under the control of TSH hormone released from pituitary gland in the brain. The thyroid gland releases certain hormones such as T3 (triiodothyronine) and T4 (thyroxine) in the blood stream due to action of TSH. The various functions of these hormones are-

1. Maintain body temperature

2. Regulate weight

3. Influence metabolism

4. Influence growth and brain development (during childhood)

 

T4 (thyroxine) hormone is present in two forms-bound T4 and free T4.the bound T4 is attached to proteins (does not enter body tissues) and freeT4 is unbound to proteins (freely enters body tissues). This test measures the unbound form of T4 which is higher in accuracy. This test is usually done along with TSH and T3 levels test.

Why is Free Thyroxine (Ft4) test prescribed?

Your health care provider may prescribe this test to look for how well thyroid gland is functioning in cases like:

1. Diagnose hypothyroidism (also called underactive thyroid)-presence of symptoms like constipation, weight gain, hair loss, irregular periods, depression, low energy dry skin etc

2. Diagnose hyperthyroidism (also called overactive thyroid)-presence of symptoms like weight loss, anxiety, tremors in hand, palpitation, difficulty sleeping, sweating, loose motions ,menstrual irregularities etc

3. To monitor response to treatment of hypothyroidism or hyperthyroidism

4. as a part of routine health check up or if there is a family history of thyroid disorders

5. To detect thyroid function during pregnancy

Procedure and prerequisites

The sample for thyroxine free Ft4 blood test is taken from blood by inserting a small needle into vein of your arm. A small amount of blood is then collected in a test tube or vial. You may feel some pain due to needle prick. If you are already on thyroid medications you may be asked to omit the dose on the day of test. Test result is affected by certain medications like Phenobarbital and chronic medical diseases like renal failure and liver cirrhosis. Hence consult your physician before undergoing this test.

Reference range

Free T4 normal range-0.8 to 1.8 nanograms/dl

High free T4 levels is seen in hyperthyroidism (overactive thyroid).it may be due to autoimmune condition such as graves’ disease.

Low free T4levels is seen in hypothyroidism (underactive thyroid). It may be due autoimmune condition like Hashimoto thyroiditis.