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T4 Free & TSH-Ultrasensitive

Also referred as

Free Thyroxine & Thyrotropin

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For men & women

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2 tests

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The T4 Free & TSH-Ultrasensitive test measures the amount of free form of thyroxine (T4 Free or FT4) and thyroid stimulating hormone (TSH) in the blood. This test helps evaluate how well your thyroid gland works and detects possible thyroid disorders, including hyperthyroidism (too much thyroid hormones) and hypothyroidism (inadequate secretion of thyroid hormones).

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Understanding T4 Free & TSH-Ultrasensitive

Thyroid-stimulating hormone (TSH), also known as thyrotropin, is produced by the pituitary gland present in the brain. TSH stimulates the thyroid gland to release thyroid hormones triiodothyronine (T3) and thyroxine (T4). Together, TSH, T3 & T4 hormones play a vital role in controlling how your body uses and stores energy and maintaining body weight and temperature.

T3 and T4 hormones exist in the blood in two forms: bound (attached to transport proteins) and free (not attached to proteins and can enter and affect the body tissues). The T4 Free & TSH-Ultrasensitive test measures the free form of T4 (FT4), a more accurate indicator of thyroid gland function than the bound form T4.

Your doctor may advise the T4 Free & TSH-Ultrasensitive test if you show symptoms suggestive of hyperthyroidism, such as increased appetite, weight loss, sweating, anxiety, tremors, etc., or hypothyroidism, such as fatigue, weight gain, dry skin, constipation, etc. This test is also recommended for women during pregnancy to check if they have adequate amounts of thyroid hormones, as these hormones are necessary for the healthy development of babies.

Abnormal levels of FT4 or TSH may indicate thyroid dysfunction (Hyperthyroidism or hypothyroidism), with high or low levels suggesting different types of thyroid disorders, such as goiter, Graves’ disease, or Hashimoto’s thyroiditis.

No special preparations are needed for this test. Usually, you do not need to stop taking your thyroid medication before taking this test. However, inform your doctor about all your medicines, as some can affect thyroid function.

Test result ranges are approximate and may differ slightly between 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 an overall treatment plan.

What does T4 Free & TSH-Ultrasensitive measure?

Contains 2 tests

The T4 Free & TSH-Ultrasensitive test measures the free form of thyroxine (FT4) hormone along with TSH to analyze how well your thyroid gland is functioning. This test has the advantage of getting a more precise analysis of your thyroid function and early detection of potential thyroid problems, thereby facilitating better treatment outcomes.

Thyroxine - Free (FT4)

The Thyroxine - Free (FT4) test measures the free form of thyroxine (T4) hormone made by the thyroid gland. T4 hormone exists in the blood in two forms: bound (attached to proteins) and free (not attached to proteins). Most of the T4 hormone circulating in the blood is bound to proteins and only a small part is free. A delicate balance of bound and free T4 should be maintained to ensure the proper functioning of the body.

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TSH (Thyroid Stimulating Hormone) Ultrasensitive

The TSH (Thyroid Stimulating Hormone) Ultrasensitive test measures the levels of TSH hormone in the blood. TSH is produced by the pituitary gland located in the brain. Its function is to stimulate and regulate the functioning of the thyroid gland. It signals the thyroid gland to increase or decrease the production of thyroid hormones T3 and T4 when their levels are low or high, respectively. Therefore, when the levels of T3 & T4 decrease, the pituitary gland is stimulated to release TSH. This high TSH level, in turn, stimulates the thyroid gland to release more thyroid hormones (T3 & T4); vice versa happens when the levels of thyroid hormones increase.

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FAQs related to T4 Free & TSH-Ultrasensitive

Why is the free T4 and TSH test done?

The free T4 and TSH test is done to check for hyperthyroidism or hypothyroidism when you are suspected of symptoms like weight gain/loss, fatigue, dry skin, hair thinning, slow heart rate, muscle weakness, anxiety, trouble sleeping, or frequent bowel movement.

Can I take the free T4 and TSH test during pregnancy?

Yes, you can take the free T4 and TSH 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 free T4 and TSH test?

No, fasting is not needed before taking this test. However, your doctor will guide you if any special preparation is required.

Is there any risk associated with this test?

There is usually no risk associated with this test. However, some people may feel a slight sting while the needle is pricked and may experience some bleeding that is normal and usually not bothersome. Some people may rarely experience slight swelling or bruising at the site of the needle prick. Applying ice 3-4 times daily for about a week may help reduce swelling and bruising. If it does not get better, consult your doctor.

What factors can lead to a variation in thyroid hormone levels?

TSH levels may show marked variation depending on the time when they are measured. So, daytime testing is usually preferred. Elevated TSH levels have also been observed with aging and during the cold winter months. Total T3 & T4 concentrations are also altered by changes in thyroid-binding protein levels. In such cases, levels of free-form T4 and T3 may be preferred.

What is Hypothyroidism?

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

Symptoms of hypothyroidism include 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.

What is Hyperthyroidism?

Hyperthyroidism (overactive thyroid gland) is a condition in which 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, palpitations, sudden weight loss, nervousness, anxiety, irritability, tremors (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.

Content created by

Dr. Swati Gupta (Anand)

MBBS, MD Lab Medicine

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Last Updated

13th Oct 2025