This is a test used to help diagnose hyperthyroidism. It is done if a patient has an abnormal TSH or T4 result or has symptoms of hyperthyroidism.

This test measures the amount of triiodothyronine, or T3, in the blood. T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4). The thyroid gland is a small butterfly-shaped organ that lies flat across your windpipe. The hormones it produces control the rate at which the body uses energy.

If the thyroid gland produces excessive amounts of T4 and T3, then the patient may have symptoms associated with hyperthyroidism, such as nervousness, tremors of the hands, weight loss, insomnia, and puffiness around dry, irritated eyes. In some cases, the patient's eyes cannot move normally and they may appear to be staring. In other cases, the patient's eyes may appear to bulge.

If the thyroid gland produces insufficient amounts of thyroid hormones, then the patient may have symptoms associated with hypothyroidism and a slowed metabolism, such as weight gain, dry skin, fatigue, and constipation. Blood levels of hormones may be increased or decreased because of insufficient or excessive production by the thyroid gland, due to thyroid dysfunction, or due to insufficient or excessive TSH production related to pituitary dysfunction.


A blood sample is obtained by inserting a needle into a vein in the arm.


24%-34% or 24-34 AU (arbitrary units; SI units)

Ranges for normal findings may vary among different laboratories and hospitals. You should always check with your doctor after having lab work or other tests done to discuss the meaning of your test results and whether your values are considered within normal limits.


Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests if necessary.


It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.