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The free T4 test measures the amount of free (unbound) thyroxine hormone in the blood. Thyroxine is produced by the thyroid gland and it functions in regulating the body’s metabolism. The test is often done to diagnose overactive (hyperthyroidism) or underactive (hypothyroidism) thyroid gland or monitor thyroid replacement therapy.
The free T4 test is a simple blood test collected via venipuncture. The levels are often reported as nanograms per deciliter (ng/dL). Normal free T4 level is between 0.8 and 2.8 ng/dL.
Higher than normal free T4 levels indicate hyperthyroidism, which can result from:
- Graves disease
- Overconsuming thyroid hormones
- Toxic goiter or thyroid nodules
- Overconsuming or overexposure to iodine
Lower than normal free T4 levels indicate hypothyroidism, which can result from:
- Acute illness or infection
- Malnutrition or fasting
- Use of certain medicines (such as lithium and amiodarone)
Am I required to fast for this test?
No, you do not need to fast for this test. However, if you are concurrently having another test done that requires fasting, you need to fast for that test as indicated.
What are normal levels?
Normally, free T4 levels are between 0.8 and 2.8 ng/dL. The normal values can vary depending on the laboratory. The exact range will be indicated on your report.
What are abnormal levels?
Lower than normal free T4 levels indicate primary hypothyroidism (failure of the thyroid to produce enough thyroxine). Higher than normal free T4 levels indicate primary hyperthyroidism (overproduction of thyroxine by the thyroid gland). Pregnancy and kidney and liver disease can also affect the test results.
Why do I need it?
If you have signs and symptoms of thyroid dysfunction, a lump or a mass in your thyroid gland, or problems of becoming pregnant, you may consider taking this test.
What type of doctor should I see if results are abnormal?
See your primary health care provider who may refer you to a specialist, such as an endocrinologist.