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Thyroxine - Total

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Overview
Interpreting Results
FAQ's
Thyroxine - Total

Overview of T4

What is T4?

Thyroxine (T4) is a hormone secreted by the thyroid gland (small, butterfly-shaped gland located in front of the neck). The T4 hormone is essential for growth and metabolism. T4 circulates in the blood in two forms: Free form (FT4) and Bound form. Thyroxine (T4) Total test measures the total levels (both free and bound forms) of Thyroxine hormone in the blood.

Why is T4 done?


  • To diagnose any suspected thyroid disease (Hypothyroidism or Hyperthyroidism)

  • To monitor treatment in patients with thyroid diseases







What does T4 Measure?

Thyroxine (T4) Total test measures the total levels (both free and bound forms) of Thyroxine hormone (T4) in the blood.

The thyroid gland secretes the following hormones:

  • Triiodothyronine (T3)

  • Thyroxine (T4)

Thyroid Stimulating Hormone (TSH), also called Thyrotropin is a hormone secreted into the blood by pituitary gland (a gland present in the brain). It directs the thyroid gland to produce and release the thyroid hormones (T3 & T4) into your blood. The iodine from the food stimulates the thyroid gland to make the thyroid hormones.

The thyroid hormones regulate growth and metabolism. If the thyroid gland produces very high amounts of these hormones (T3 and T4), symptoms of weight loss, rapid heartbeat, tremors, sweating, anxiety, increased sensitivity towards heat, etc occurs. This is known as Hyperthyroidism.

The decreased production of thyroid hormones (T3 and T4) results in Hypothyroidism which may lead to weight gain, fatigue, slow heart rate, increased sensitivity towards 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 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). TBG is also called Thyroid hormone Binding Globulin (THBG).

2) Free form - It enters the body tissues where it is needed and this is the active form.

Hence, the T4 hormone can be measured as Free T4 or Total T4. The total T4 includes both bound and free forms circulating in the blood and can be affected by the amount of protein available in the blood to bind to them.

While the total T4 test is a useful indicator of T4 levels in the presence of normal binding proteins, it is not useful when levels of binding proteins are increased or decreased. For example, increased total T4 levels can be seen despite normal levels of free T4 levels and normal thyroid function due to an increase in thyroxine-binding proteins.

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


Preparation for T4

  • Do not eat or drink anything other than water for 8-12 hours before the test.

Sample Type for T4

The sample type collected for Thyroxine - Total is: Blood

Interpreting T4 results

Interpretations

Values higher than normal reference range are seen in hyperthyroidism and patients with acute thyroiditis.

Values lower than normal reference range are seen in hypothyroidism, myxedema, cretinism, chronic thyroiditis, and occasionally, subacute thyroiditis.





Answers to Patient Concerns & Frequently Asked Questions (FAQs) about T4

Frequently Asked Questions about Thyroxine - Total

Q. What is Hyperthyroidism?
Hyperthyroidism is a condition in which the thyroid gland is overactive. This leads to 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, overconsumption of iodine, and excess of synthetic thyroid hormone. Diagnosis of this condition can help relieve the symptoms and prevent long-term complications.
Q. 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.
Q. 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 low intake of iodine in diet or Hashimoto's thyroiditis (an autoimmune disease). Other 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 improper functioning of thyroid gland since birth.
Q. What are the symptoms of Hypothyroidism?
Symptoms of Hypothyroidism include: Tiredness (fatigue), constipation, feeling cold, dry skin, weight gain, muscle weakness, decreased sweating, depression, slower 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, hoarseness, puffy face, dry and thin hair.
Q. How is this test performed?
This test is performed on a blood sample. A syringe with a fine needle is used to withdraw blood sample from a blood vessel in your arm generally from the inner side of the elbow area. The doctor, nurse or the phlebotomist will tie an elastic band around your arm which will help the blood vessels to swell with blood. This makes it easier to withdraw blood. You may be asked to tightly clench your fist. Once the veins are clearly visible, the area is cleaned with an antiseptic solution and then the needle is inserted into the blood vessel to collect the sample. You may feel a tiny pinprick during the procedure. Blood sample once collected is then sent to the laboratory.
Q. 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.
Q. 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.
Q. 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.
Q. What tests your doctor may order to detect the thyroid disease in addition to T4 Total test?
Your doctor may order the following tests: Thyroid stimulating hormone (TSH) test, Thyroxine (T4) Free hormone test, Thyroid Hormone Binding Globulin (THBG) and Thyroid profile.
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