Free T4 test
Free thyroxine test; Thyroxine test by equilibrium dialysisT4 (thyroxine) is the main hormone produced by the thyroid gland. A laboratory test can be done to measure the amount of free T4 in your blood. Free T4 is the thyroxine that is not attached to a protein in the blood.
How the Test is Performed
A blood sample is needed.
Blood sample
Venipuncture is the collection of blood from a vein. It is most often done for laboratory testing.
Read Article Now Book Mark ArticleHow to Prepare for the Test
Your health care provider will tell you if you need to stop taking any medicines that may affect the test result. In general, test results are not affected by other medicines you may be taking. However, certain supplements including biotin (vitamin B7) can affect the results. Tell your provider if you are taking biotin.
Pregnancy and some diseases, including kidney and liver disease, can also affect the results of this test.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
Why the Test is Performed
Your provider may recommend this test if you have signs of a thyroid disorder, including:
- Abnormal findings of other thyroid blood tests, such as TSH or T3
TSH
A TSH test measures the amount of thyroid stimulating hormone (TSH) in your blood. TSH is produced by the pituitary gland. It prompts the thyroid g...
Read Article Now Book Mark ArticleT3
Triiodothyronine (T3) is a thyroid hormone. It plays an important role in the body's control of metabolism (the many processes that control the rate...
Read Article Now Book Mark Article - Symptoms of an overactive thyroid
Overactive thyroid
Hyperthyroidism is a condition in which the thyroid gland makes too much thyroid hormone. The condition is often called overactive thyroid.
Read Article Now Book Mark Article - Symptoms of an underactive thyroid
Underactive thyroid
Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone. This condition is often called underactive thyroid....
Read Article Now Book Mark Article - Hypopituitarism -- when the pituitary gland does not produce enough of its hormones, including the hormone which stimulates the thyroid
- Lump or nodule in the thyroid
Lump or nodule
A thyroid nodule is a growth (lump) in the thyroid gland. The thyroid gland is located at the front of the neck, just above where your collarbones m...
Read Article Now Book Mark Article - Enlarged or irregular thyroid gland
- Problems becoming pregnant
This test is also used to monitor people who are being treated for thyroid problems.
Normal Results
A typical normal range is 0.8 to 1.9 nanograms per deciliter (ng/dL), or 10.3 to 24.5 picomoles per liter (pmol/L).
Normal value ranges may vary slightly among different laboratories. Some laboratories use different measurements or may test different specimens. Talk to your provider about the meaning of your specific test results.
The normal range is based on a large population and is not necessarily normal for an individual. You may be having symptoms of hyperthyroidism or hypothyroidism even though your free T4 is in the normal range. The TSH test may help determine if your symptoms are related to thyroid disease. Talk to your provider about your symptoms.
What Abnormal Results Mean
To fully understand results of the free T4 test, results of other thyroid blood tests, such as TSH or T3, may be needed.
Test results may also be affected by pregnancy, estrogen level, liver problems, more severe body-wide illnesses, and inherited changes in a protein that binds T4.
A higher than normal level of T4 may be due to conditions that involve an overactive thyroid, including:
- Graves disease
Graves disease
Graves disease is an autoimmune disorder that leads to an overactive thyroid gland (hyperthyroidism). An autoimmune disorder is a condition that occ...
Read Article Now Book Mark Article - Taking too much thyroid hormone medicine
- Thyroiditis
- Toxic goiter or toxic thyroid nodules
- Some tumors of the testes or ovaries (rare)
- Getting medical imaging tests with contrast dye that contains iodine (rare, and only if there is a problem with the thyroid)
- Eating a lot of foods that contain iodine (very rare, and only if there is a problem with the thyroid)
A lower than normal level of T4 may be due to:
- Hypothyroidism (including Hashimoto disease and other disorders involving an underactive thyroid)
Hypothyroidism
Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone. This condition is often called underactive thyroid....
Read Article Now Book Mark Article - Severe acute illness
- Malnutrition or fasting
Malnutrition
Malnutrition is the condition that occurs when your body does not get enough nutrients.
Read Article Now Book Mark Article - Use of certain medicines
Risks
There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but may include:
- Excessive bleeding
- Fainting or feeling lightheaded
- Multiple punctures to locate veins
- Hematoma (blood buildup under the skin)
Hematoma
A bruise is an area of skin discoloration. A bruise occurs when small blood vessels break and leak their contents into the soft tissue beneath the s...
Read Article Now Book Mark Article - Infection (a slight risk any time the skin is broken)
References
Faix JD. Thyroid function testing (thyrotropin, triiodothyronine, and thyroxine). In: Robertson RP, ed. DeGroot's Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 68.
Guber HA, Oprea M, Russell YX. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 25.
Pearce EN, Hollenberg AN. Thyroid. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 207.
Salvatore D, Cohen R, Kopp PA, Larsen PR. Thyroid pathophysiology and diagnostic evaluation. In: Melmed S, Auchus RJ, Golfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 11.
Wassner AJ, Smith JR. Thyroid development and physiology. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 579.
Blood test - illustration
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. Preparation may vary depending on the specific test.
Blood test
illustration
Blood test - illustration
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. Preparation may vary depending on the specific test.
Blood test
illustration
Review Date: 2/28/2024
Reviewed By: Sandeep K. Dhaliwal, MD, board-certified in Diabetes, Endocrinology, and Metabolism, Springfield, VA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.