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Free T4

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Also known as: T4; Thyroxine
Formal name: Free Thyroxine

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At a Glance

Why Get Tested?

To help evaluate thyroid gland function; to help diagnose thyroid disease; to screen for hypothyroidism in newborns; to monitor effectiveness of treatment

When to Get Tested?

When you have signs and symptoms of thyroid disease, usually after an abnormal result on a TSH test; commonly performed as a screening test on newborns soon after birth; when you are being treated for a thyroid disorder

Sample Required?

A blood sample drawn from a vein in your arm or from pricking the heel of an infant

Test Preparation Needed?

None needed; however, certain medications can interfere with the free T4 test, so tell your healthcare provider about any drugs that you are taking. If you take thyroid hormone as treatment for thyroid disease, it is recommended that your blood sample be drawn before you take your dose for that day.