Free Triiodothyronine (FT3) Test

What is Free Triiodothyronine (FT3)?

Triiodothyronine or T3 is a hormone secreted by the thyroid glands and is important for the metabolism of your body including the heart rate and temperature. About 20% of T3 is secreted by the butterfly-shaped thyroid glands; the rest 80% is formed by the peripheral conversion (deiodination) of another thyroid hormone T4 (thyroxine). The hormone T3 is four times more potent than T4. The levels of both T3 and T4 are regulated via a negative feedback mechanism involving 3 glands- thyroid, pituitary and the hypothalamus.

Thyrotropin-releasing hormone (hypothalamus)—-Thyroid
stimulating hormone (pituitary)— Triiodothyronine, Thyroxine (thyroid).

As is evident from this axis, a defect in any of these glands can lead to a disorder of the thyroid function, leading to a deficiency or an excess of these vital hormones.

What is measured by a Free Triiodothyronine (FT3) Test?

Most of T3 circulates in the body in the bound form-TBG (thyroid-binding globulin) being the major binding protein. Albumin and pre-albumin are the other 2 proteins that bind T3. Only 0.2-0.4% of T3 is found in the free-from in the body. It is this unbound fraction that is measured by the FT3 test.

Why has my doctor ordered FT3 Test for me?

A free triiodothyronine test can be used to diagnose as well as monitor the response to therapy for thyroid disorders. Your physician may order a free triiodothyronine test for you in order to-

– Rule out T3 thyrotoxicosis,
– Estimate the efficacy of thyroid replacement therapy,
– Spot protein-binding anomalies.

What are the signs and symptoms of thyrotoxicosis?

Thyrotoxicosis results due to the increased levels of thyroid hormones leading to an imbalance of the body’s metabolism. Around 2% of females and 0.2% of males suffer from this disorder. The main clinical features are a consequence of the overactivity of the body’s sympathetic system. Common symptoms of thyrotoxicosis include the following:

– Nervousness/anxiety
– Unexplained weight loss
– Increased perspiration
– Heat intolerance
– Irregular menses (in women)
– Palpitations
– Tachycardia
– Tremor
– Fatigue
– Lid lag

Subclinical hyperthyroidism is a state in which the levels of TSH (thyroid-stimulating hormone) are low in the blood but the levels of both T3 and T4 are normal. In such cases, the classical signs of thyrotoxicosis may be absent.

Cardiovascular disorders, especially atrial fibrillation is, however, very common in this disorder. Other manifestations of subclinical hyperthyroidism include osteoporosis and hypercalcemia.

What are the causes of elevated FT3?

Elevated FT3 levels are seen in hyperthyroidism (a disorder when the thyroid glands secrete excess amounts of T3 and T4). The common causes of hyperactive thyroid glands include- Toxic adenoma,
Toxic multinodular goiter, Subacute thyroiditis and Diffuse toxic goiter (commonly known as Grave’s disease). Certain medicines can also lead to high FT3 levels. These include-

– Exogenous thyroxine/levothyroxine
– Amiodarone
– Aspirin
– Anti-epileptics
– Ranitidine
– Heparin
– Iodine containing contrast agents.

People residing at a high altitude tend to have higher levels of FT3.

What are the causes of decreased FT3?

Low FT3 levels are seen in hypothyroidism (a disorder when the thyroid glands secrete less amount of T3 and T4). Starvation may also lead to low levels of FT3 in your blood. Certain medicines can also lead to low FT3 levels. These include –

– Steroids
– Radioisotopes
– Somatostatin
– Methimazole
– Propranolol

How should I prepare for an FT3 Test? What are the precautions that I need to undertake?

No specific preparation is required. You need to discuss with the physician, your medical history. As an FT3 test is a blood test and involves the standard techniques of venipuncture, you need to inform your physician if-

– You are taking any blood thinners
– You are suffering from any coagulopathy
– You are allergic to any drug.

You also need to inform your doctor if you are pregnant or suffering from any liver disease because both these conditions can interfere with the result of the FT3 test. Provide details about any medications that you may be consuming at the time of the test as many medications have the potential to increase or decrease the FT3 levels in your blood.

Do I need to fast overnight?

Yes, overnight fasting of 10-12 hours is required before a free triiodothyronine test.

Can I undergo an FT3 Test if I am pregnant?

Yes, you can undergo an FT3 test if you are pregnant or breastfeeding your child. It is a safe, risk-free test with no potential hazard to a gravid female or to her unborn fetus.

Can children undergo an FT3 Test?

Yes, children can safely undergo the FT3 test as it is a simple, safe and risk-free procedure.

How is the actual FT3 Test performed?

It is a blood test carried out on an outpatient basis unless you are already admitted to a hospital. You will be directed to the collection area of the laboratory or the hospital which is dedicated to the purpose of accumulating blood samples for testing.

– You will be asked to sit on a comfortable chair or lie down on a hospital bed/examination table. You will be asked to roll up your sleeves.

– A technician will apply a tourniquet proximally to the site chosen for
venipuncture. Generally, the elbow region of the non-dominant arm is chosen. The median cubital vein-a superficial vein lying in the region of cubital fossa is the preferred vessel. You may be asked to form a
fist.

– This makes the veins more prominent, allowing easier access. The area is cleaned with a disinfectant. It is allowed to dry.

– A sterile, single-use needle is used to puncture the vein and draw about 5 ml of blood. The pressure is applied to the vein with sterile gauze after the needle is withdrawn.

– The tourniquet is released. The blood sample is collected in a vial.

How long does the procedure last?

The entire procedure is over in a few minutes. In obese patients, patients belonging to the geriatric age group and very young children, venous access is sometimes difficult, leading to more than one prick.

How long does it take for the result of an FT3 test to be out?

The results of the Free Triiodothyronine (FT3) Test are released the same day; normally the turnaround time is about 6 hours.

What are the risks involved?

As is the case with any procedure involving venipuncture, you will feel a stinging sensation when the needle punctures your vein. The pain perception varies in different individuals. Bleeding, bruising and inflammation of the vein may be encountered. You may also experience mild nausea and dizziness at the sight of blood. It is advisable to look away during the procedure if you are faint-hearted. An infection at the site of the needle entry can be a late complication. Consult your physician if you experience pus or an abscess at that site.

What do the results of my test convey?

The normal range of FT3 in a healthy adult is – 3.5 – 6.5 pmol/L. This is also known as the euthyroid range for FT3. A higher than normal value points toward the diagnosis of thyrotoxicosis. Free triiodothyronine levels are not of much use in making a diagnosis of hypothyroidism.

A free triiodothyronine test cannot be used as an independent entity to make a diagnosis of hyperthyroidism. It is used to confirm the diagnosis supplementing the already carried out assays of T4,TSH and total T3. The most accurate and reliable test for diagnosing thyrotoxicosis is the TSH test. Its value is almost unmeasurable in conditions of thyrotoxicosis.

What are the advantages and limitations of the FT3 test?

What is the cost of an FT3 test in India?

The cost of performing a free T3 test in India ranges from Rs 400-750 ; the actual cost depending on the city you reside in.

What are the other thyroid function tests?

The other routinely performed thyroid function tests include-
– TSH test
– FT4 (free thyroxine) test
– Total T3 or T4 test
– Autoantibody tests- include ELISA test for anti-TPO antibody.
– Scintigraphy- thyroid gland scan using radioactive iodine or technetium.