Serum free triiodothyronine measurement

This test measures the amount of free triiodothyronine (T₃) in blood. It is used to evaluate and manage disorders of the thyroid gland.

Laboratory tests may be done for many reasons. Tests are performed for routine health screenings or if a disease or toxicity is suspected. Lab tests may be used to determine if a medical condition is improving or worsening. Lab tests may also be used to measure the success or failure of a medication or treatment plan. Lab tests may be ordered for professional or legal reasons. You may need this test if you have:

• Elevated thyroid hormone
• T3 toxic hyperthyroidism

Venous and capillary blood:

Before having blood collected, tell the person drawing your blood if you are allergic to latex. Tell the healthcare worker if you have a medical condition or are using a medication or supplement that causes excessive bleeding. Also tell the healthcare worker if you have felt nauseated, lightheaded, or have fainted while having blood drawn in the past.

Tell the person doing the test if you are pregnant, and what pregnancy trimester you are in at the time of the test.

If you use thyroid medications, tell the person doing the test what medications you use, and when your last dose was taken.

Umbilical cord blood:

Ask the healthcare worker for information about how to prepare for this test.

A sample of venous blood or umbilical cord blood may be collected for this test.

Venous blood:

Common sites to collect a capillary blood sample are the fingertip and earlobe. Infants often have a capillary blood sample taken from the heel of the foot. Once the site is selected, the healthcare worker may heat the area with a warm compress to ensure adequate blood flow. The area will be cleaned with antiseptic. A small needle is used to make a cut in the skin surface, and the area may be squeezed gently to produce blood. The blood is collected in small collection device.

Umbilical cord blood:

To collect an umbilical cord blood sample after an infant is born, the healthcare worker may use a needle and syringe to draw blood from the umbilical cord while the cord is still attached to the infant. Blood samples may also be collected from the part of the umbilical cord that has been detached from the infant.

After birth, an infant's body does not need the attached umbilical cord stump or its blood vessels, but they may be used temporarily for medical purposes. If the infant has a catheter inserted in a vessel of the umbilical cord, the blood sample may be collected through the existing catheter.

The amount of discomfort you feel will depend on many factors, including your sensitivity to pain. Communicate how you are feeling with the person doing the test. Inform the person doing the test if you feel that you cannot continue with the test.

Venous blood:

During a blood draw, you may feel mild discomfort at the location where the blood sample is being collected.

Umbilical cord blood:

There are several different ways that a cord blood sample may be collected. Depending on the procedure used to obtain the sample, the test may be uncomfortable. Ask the healthcare worker to explain how the test may feel.

Venous blood:

After a blood sample is collected from your vein, a bandage, cotton ball, or gauze may be placed on the area where the needle was inserted. You may be asked to apply pressure to the area. Avoid strenuous exercise immediately after your blood draw. Contact your healthcare worker if you feel pain or see redness, swelling, or discharge from the puncture site.

Umbilical cord blood:

Depending on the procedure used to obtain a sample of cord blood, there may be special instructions for you to follow. Ask the healthcare worker for any special instructions following this procedure.

You have the right to help plan your care. To help with this plan, you must learn about your health condition and how it may be treated. You can then discuss treatment options with your caregivers. Work with them to decide what care may be used to treat you. You always have the right to refuse treatment.