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Thyroxine - Total (T4)

Also referred as

Total Thyroxine

Total T4

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The Thyroxine - Total (T4) is a blood test that measures the level of total thyroxine (T4) hormone in the blood. This test helps assess overall thyroid function and detect possible thyroid disorders. It is also performed in combination with total T3 and TSH tests as part of a thyroid profile total test.

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Understanding Thyroxine - Total (T4)

Thyroxine, also known as T4, is a hormone produced by the thyroid gland. The thyroid is a small, butterfly-shaped gland located at the front of your neck. It helps regulate the body’s metabolism, normal temperature, blood pressure, and heart rate. T4 along with triiodothyronine (T3), another hormone produced by the thyroid gland, plays an important role in the growth and development of the nervous system and the brain. T4 exists in the blood in two forms: bound (attached to proteins) and free (not attached to proteins).

The Thyroxine - Total (T4) measures the total level (both free and bound forms) of the T4 hormone. Deranged levels of T4 can result in either hyperthyroidism (the thyroid gland produces too much T4) or hypothyroidism (the thyroid gland produces less T4). Your doctor may suggest this test if you show symptoms of hyperthyroidism, like increased appetite, weight loss, sweating, anxiety, tremors, etc., or hypothyroidism, like fatigue, weight gain, dry skin, constipation, etc.

No special preparations are needed for the Thyroxine - Total (T4) test. However, do let your doctor know about all the medications you take as some of those can affect thyroid function. Biotin (Vitamin B7) is a commonly taken over-the-counter supplement that can lead to some aberrancy in your thyroid function test values even if they are normal. Therefore, avoid taking biotin 2 days before this test.

Test result ranges are approximate and may differ slightly between different 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, make recommendations for lifestyle modifications such as diet and exercise, evaluate whether or not medication will be required to manage your condition, and formulate your overall treatment plan.

What does Thyroxine - Total (T4) measure?

The Thyroxine - Total (T4) test measures both the bound and unbound/free form of thyroxine (T4) hormone in the blood. T4 exists in the blood in two forms: bound (attached to proteins) and free (not attached to proteins). Most of the T4 circulating in the blood is bound to proteins and only a small part is free. It is necessary to maintain a fine balance of these forms to ensure the proper functioning of the body.

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FAQs related to Thyroxine - Total (T4)

Why is the Thyroxine - Total (T4) test done?

The Thyroxine - Total (T4) test is done to measure the level of T4 in the body and evaluate symptoms suggestive of an imbalance in this hormone, such as weight gain/loss, fatigue, dry skin, hair thinning, slow heart rate, or depression, muscle weakness, anxiety, trouble sleeping, or frequent bowel movement.

Can I take the Total T4 test during pregnancy?

Yes, you can take the Total T4 test during pregnancy. This test along with others is usually recommended during pregnancy to measure the levels of thyroid hormones, as thyroid disorders can complicate pregnancy.

Is fasting required for the Total 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 the Total T4 levels are high?

High Total T4 levels can lead to a condition called hyperthyroidism in which the thyroid gland produces too much thyroid hormone. Some of the common symptoms of hyperthyroidism include weight loss, nervousness, irritability, increased sweating, and rapid heartbeat.

What happens when the Total T4 levels are low?

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

What are the factors affecting thyroid hormones?

Changes in thyroid hormones are typically associated with concordant changes in T3, T4 and TSH levels. Unexpectedly abnormal or discordant thyroid test values may be seen with some rare, but clinically significant conditions such as hypothyroidism (inadequate thyroid hormones), TSH-secreting pituitary tumors, thyroid hormone resistance, or the presence of heterophilic antibodies (HAMA) or thyroid hormone autoantibodies.

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.

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.

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Last Updated

12th Oct 2025