What is being tested?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. Their production is regulated by a
feedback system. When blood levels of thyroid hormones decline, the hypothalamus (an organ in the brain) releases thyrotropin releasing hormone, which stimulates the pituitary (a tiny organ below the brain and behind the sinus cavities) to produce and release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce and/or release more thyroid hormones. Most of the thyroid hormone produced is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues.
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.
About 99.7% of the T3 found in the blood is attached to a protein (primarily thyroxine-binding globulin but also several other proteins) and the rest is free (unattached). Separate blood tests can be performed to measure either the total (both bound and unattached) or free (unattached) T3 hormone in the blood.
