Thyroxine (T4) Test

The thyroxine (T4) test is a blood test used to measure the level of thyroxine, a hormone produced by the thyroid gland, in the bloodstream.

Purpose of the Test

The T4 test is used to:

• Diagnose and monitor thyroid disorders, such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid)
• Evaluate the effectiveness of treatment for thyroid disorders
• Monitor thyroid function in people with a history of thyroid problems or those taking medications that can affect thyroid function

How the Test is Performed

A healthcare professional will draw a blood sample from a vein in the arm, usually using a needle and syringe or a vacuum tube.

Types of T4 Tests

There are two types of T4 tests:

• Total T4 (TT4) test: measures the total amount of thyroxine in the blood, including both bound and free T4
• Free T4 (FT4) test: measures only the free thyroxine in the blood, which is the active form of the hormone

Interpretation of Results

The results of the T4 test are typically interpreted as follows:

• Normal range: varies depending on the laboratory and the individual's age, but generally falls between 4.6 and 12.0 μg/dL for TT4 and 0.8 and 2.7 ng/dL for FT4
• Low T4 levels: may indicate hypothyroidism or other conditions such as pituitary gland problems
• High T4 levels: may indicate hyperthyroidism, thyroiditis, or other conditions such as Graves' disease

Risks and Limitations

The T4 test is generally a safe and low-risk procedure, but may cause:

• Pain or bruising at the needle site
• Dizziness or lightheadedness
• Infection (rare)

The T4 test has some limitations, including:

• May not accurately reflect thyroid function in people with certain medical conditions, such as severe illness or liver disease
• May be affected by certain medications, such as estrogen and testosterone