Reasons, Preparation and Procedure To Conduct a T4 (Thyroxine) Lab Test

Submitted on March 27, 2012

T4 (Thyroxine) Lab Test

The amount of T4 hormone present in the blood is measured by using a thyroxine or T4 test. An individual's growth and metabolic rate is determined by the presence of this hormone that is secreted by the thyroid glands. Doctors may recommend a T4 lab test or a T4 blood test to assess thyroid functioning. Two blood tests are performed as part of this test. These are Total T4 and Free T4.

The Total T4 test studies the total amount of thyroxine present in the blood, this includes thyroxine attached to blood proteins responsible for transporting the hormone in and through the bloodstream. The Free T4 test studies only thyroxine that remains unattached to proteins.

Reasons Why It is Conducted

Doctors often recommend T4 testing if they suspect any thyroid disorder. For example, hyperthyroidism is a condition wherein the thyroid is overactive. Symptoms usually noted are weight loss, sweating, and fast heart rate. Hypothyroidism is another thyroid disorder, wherein the patient suffers from an underactive thyroid. Symptoms associated with this disorder are weight gain, feeling cold, and fatigue. Newborn children are mostly screened for hypothyroidism, which if left untreated can lead to mental disabilities.


An elastic band is tied at the elbow. This acts as a tourniquet, making the vein below the band to swell. This makes it easier to draw out blood from the vein. The healthcare provider will then clean the area with an antiseptic ointment. A needle is then inserted into the vein and blood is collected in a vial or syringe attached to the needle. After collecting the blood sample, a cotton piece or a bandage is placed on the puncture site to stop the bleeding, the needle is then gently removed and a small tape is applied to the puncture site. Normal values differ slightly across laboratories however, a reading between 4.5 to 11.2 mcg/dL or micrograms per deciliter is considered normal. Values greater than the normal range could point to out to hyperthyroid conditions that include Grave's disease, iodine-induced hyperthyroidism, germ cell tumors, early hashimoto's disease, and trophoblastic disease among others. Values lower than the normal range could point to Hypothyroidism, myxedema, goitrous diseases, cretinism, amyloid goiter, hemochromatosis or scleroderma, following neck irradiation for head and neck cancer.


There is no special preparation required for the T4 blood test. However, the doctor should be aware of all medications and dosages to avoid faulty test results. Children can be made to wear a short-sleeved shirt or t-shirts to allow the lab technician to draw the blood easily and quickly.