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Thyroxine - Free

Also referred as

Free T4

For men & women

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18 hours

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A free thyroxine test (FT4), often called a free T4 test, measures the amount of free thyroxine in your blood. T4 circulates in the blood in two forms: free form (FT4) and bound form. Thyroxine (T4) is a hormone secreted by the thyroid gland and is essential for growth and metabolism. The FT4 test helps to diagnose any suspected thyroid disease (hypothyroidism or hyperthyroidism) and to check the treatment efficacy in people diagnosed with thyroid condition.

Higher T4 levels could indicate an overactive thyroid (hyperthyroidism) and shows symptoms like excessive appetite, anxiety, heart palpitations, sweating, shortness of breath, and weight loss. Abnormally low thyroxine level leads to hypothyroidism and causes symptoms like weakness, cold intolerance, weight gain, and dry skin.

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What does Thyroxine - Free measure?

The thyroid gland secretes the following hormones:

Triiodothyronine (T3)
Thyroxine (T4)

Thyroid Stimulating Hormone (TSH) is a hormone secreted into the blood by the pituitary gland (a gland present in the brain) which tells your thyroid gland to make and release the thyroid hormones (T3 & T4) into your blood. The thyroid gland uses the iodine gained from food to make the thyroid hormones.

The thyroid hormones are essential for growth and metabolism. If the thyroid gland produces very high amounts of T3 and T4 hormones, you may experience symptoms like weight loss, rapid heartbeat, tremors, sweating, anxiety, increased sensitivity to heat, etc. This is known as hyperthyroidism.

The decreased production of thyroid hormones (T3 and T4) results in hypothyroidism which may cause weight gain, fatigue, slow heart rate, increased sensitivity to 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 hormones decrease, the pituitary gland is stimulated to release TSH. High TSH, in turn, increases the release of thyroid hormones (T3 & T4) from the thyroid gland and vice-versa.

T4 hormone constitutes about 90% of thyroid hormones and circulates in the blood in two forms:

1) Bound form - It is bound to the proteins present in the blood and this prevents it from entering the body tissues. The three main proteins in the blood that the T4 hormone is bound to are albumin, transthyretin and Thyroxine-binding globulin (TBG), also called Thyroid hormone binding globulin (THBG).

2) Free form - This is the active form which enters the body tissues where it's needed. Free Thyroxine (FT4) constitutes only 0.3% of the total T3 hormone.

Hence, the T4 hormone can be measured as Free T4 or Total T4. Total T4 includes both the bound and the free forms, circulating in the blood and can be affected by the amount of protein available in the blood to bind to them. Therefore, Thyroxine (T4) Free Test is a useful indicator of the T4 levels in the blood when binding proteins are increased or decreased.

Thyroxine (T4) Free Test is also done as a part of the Thyroid Profile Free Test which includes two more tests: Thyroid Stimulating Hormone (TSH) and Triiodothyronine (T3) Total.

FAQs related to Thyroxine - Free

Is there any preparation required before the test?

Inform the doctor about the medications you may be taking. No other specific preparations are usually required before this test.

What is Hyperthyroidism?

Hyperthyroidism is a condition in which the thyroid gland is overactive and there is an increased production of thyroid hormones by the thyroid gland. Causes of Hyperthyroidism include Graves’ disease, multinodular goiter, thyroid nodules, toxic adenoma, inflammation of the thyroid, eating too much iodine, and too much synthetic thyroid hormone. Diagnosis of this condition can help to relieve the symptoms and prevent long-term complications.

What are the symptoms of Hyperthyroidism?

Symptoms of Hyperthyroidism include: Rapid heartbeat (tachycardia) — commonly more than 100 beats/minute — irregular heartbeat (arrhythmia) or pounding of your heart (palpitations), sudden weight loss, nervousness, anxiety, irritability, tremor (usually a fine trembling in your hands and fingers), changes in menstrual patterns, increased sensitivity towards heat, changes in bowel patterns (especially more frequent bowel movements), an enlarged thyroid gland (goiter) which may appear as a swelling at the base of your neck, increased appetite, sweating, fatigue, muscle weakness, sleep disturbances, skin thinning, fine and brittle hair.

What is Hypothyroidism?

Hypothyroidism, also called underactive thyroid or low thyroid is a condition in which the thyroid gland does not produce enough thyroid hormones. It can be commonly caused by intake of low iodine diet or Hashimoto's thyroiditis (an autoimmune disease). Less common causes include previous treatment with radioactive iodine, injury to the pituitary gland which secretes TSH, intake of certain medicines, previous thyroid surgery, or a lack of a functioning thyroid gland at birth.

What are the symptoms of Hypothyroidism?

Hypothyroidism can cause symptoms such as tiredness (fatigue), hoarseness, constipation, feeling cold, dry skin, dry and thin hair, puffy face, weight gain, and muscle weakness. Along with that, one may experience decreased sweating, depression, slowed heart rate, increased blood cholesterol levels, pain, and stiffness in your joints, impaired memory, problems of infertility or menstrual changes, muscle stiffness, aches, and tenderness.

Is there any risk associated with this test?

There is no risk associated with the test. However, since this test involves a needle prick to withdraw the blood sample, in very rare cases, a patient may experience increased bleeding, hematoma formation (blood collection under the skin), bruising or infection at the site of needle prick.

How can hyperthyroidism affect a pregnant woman and her baby?

An untreated hyperthyroidism during pregnancy can lead to miscarriage, premature birth, low birth weight of the baby, preeclampsia (a very high rise in blood pressure in late pregnancy), thyroid storm (a sudden, severe worsening of symptoms of hyperthyroidism) and congestive heart failure.

How can hypothyroidism affect a pregnant woman and her baby?

An untreated hypothyroidism during pregnancy can cause preeclampsia (a very high rise in blood pressure in late pregnancy), anemia, miscarriage, low birth weight of the baby, stillbirth (death or loss of a baby before or during delivery) and rarely, congestive heart failure. These problems are noticed mostly as a result of severe hypothyroidism. The mother’s thyroid hormones play an important role in the development of the baby’s brain and nervous system. Untreated hypothyroidism, especially during the first trimester can cause low IQ and can hinder the baby’s normal development.