T4 (thyroxine) is a hormone produced by the thyroid gland. A
laboratory test can be done to measure the amount of T4 in your
blood. How the test is performed A blood sample is needed. For
information on how this is usually done, see: Venipuncture How
to prepare for the test Your health care provider will tell you,
if necessary, to stop taking drugs that may interfere with the
test. Drugs that can increase T4 measurements include: Birth
control pills Drugs that can decrease T4 measurements include:
Antithyroid drugs (for example, propylthiouracil and
methimazole) This list may not include all medications. How the
test will feel When the needle is inserted to draw blood, some
people feel moderate pain, while others feel only a prick or
stinging sensation. You may feel some throbbing after the test.
Why the test is performed This test is done to check your
thyroid function. Thyroid function is complex and depends on the
action of many different thyroid hormones, including thyroid-
stimulating hormone (TSH) and T3