Why It Is Done

* Find out what is causing an abnormal thyroid-stimulating hormone (TSH) test. This is the most common reason for thyroid hormone tests.*Check how well treatment for thyroid disease is working. The total thyroxine (T4), total  thyroxine (FT4), and total thyroxine index (FTI) values are often used to keep track of treatment for hyperthyroidism.*Determine if your thyroid is not working correctly due to hyperthyroid - an over-working thyroid.*Determine if your thyroid is not working correctly due to hypothyroid - poor thyroid function

Test Overview

Thyroid hormone tests are blood tests that check how well the thyroid gland is working. The thyroid gland makes hormones that regulate the way the body uses energy and control your body's metabolism.
The thyroid gland is a butterfly-shaped gland that lies in front of your windpipe (trachea), just below your voice box (larynx). The thyroid gland uses iodine from food to make two thyroid hormones, thyroxine (T4) and triiodothyronine (T3). The thyroid gland stores these thyroid hormones and releases them as they are needed.

How To Prepare

Many medicines may change the results of this test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. If you are taking thyroid medications, tell your doctor when you took your last dose. Your doctor may instruct you to stop taking thyroid medications temporarily before having this test.