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Alternate Names

  • thyroxine test


A T4 blood test measures the amount of a thyroid hormone known as thyroxine, also known as T4.

Who is a candidate for the test?

A T4 test is usually performed to evaluate thyroid function. The levels of thyroid hormone in the blood are important because thyroid hormone controls the body's rate of basal metabolism, which is the energy needed to keep the body functioning at rest.

How is the test performed?

A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle and is collected into a syringe or vial for T4 testing in the laboratory. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.

What is involved in preparation for the test?

Generally, there is no preparation for a T4 test.

What do the test results mean?

Normal levels of T4 range from 4.5 to 12.5 micrograms per deciliter (mcg/dL).
Abnormally high levels of T4 may be seen with disorders that stimulate the thyroid gland. These include:
  • acute thyroiditis, or inflammation of the thyroid
  • Graves' disease, a condition caused by an antibody to thyroid tissue that causes excess thyroid hormone production
  • hyperthyroidism (an overactive thyroid gland) occurring for other or unknown reasons
  • thyroid cancer
Abnormally low T4 levels may indicate the following:
  • Hashimoto's disease, an autoimmune disorder in which the body attacks the thyroid gland and interferes with thyroid hormone production
  • hypothyroidism (an underactive thyroid) for other or unknown reasons
  • kidney failure
  • protein malnutrition

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