T-3. TOTAL

Triiodothyronine

Triiodothyronine (T3) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called thyroxine (T4) and together they help control the rate at which the body uses energy. Almost all of the T3 (and T4) found in the blood is bound to protein. The rest is free (unbound) and is the biologically active form of the hormone. Tests can measure the amount of free T3 or the total T3 (bound plus unbound) in the blood.

Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues. Most of the hormone produced by the thyroid is T4. This hormone is relatively inactive, but it is converted into the much more active T3 in the liver and other tissues. T3 levels are regulated by the feedback system that the body uses to maintain stable amounts of thyroid hormones in the blood. T4 and TSH, along with its regulatory hormone thyrotropin releasing hormone (TRH), which comes from the hypothalamus, are also part of the feedback system.

When blood levels of thyroid hormones decline, the pituitary gland produces and releases TSH in response to TRH stimulation. TSH then stimulates the thyroid gland to produce and/or release more T4 and T3.     When thyroid hormone levels increase in the blood, the pituitary gland produces less TSH and the thyroid produces less T4 and T3. When all three organs (hypothalamus, pituitary and thyroid) are functioning normally, thyroid production is regulated to maintain relatively stable levels of thyroid hormones in the blood.

If the thyroid gland produces excessive amounts of T4 and T3, then the person affected may have symptoms associated with overactive thyroid (hyperthyroidism), such as nervousness, tremors of the hands, weight loss, insomnia, and puffiness around dry, irritated eyes and in some cases, bulging eyes. Graves disease is the most common cause of hyperthyroidism. (See the article on Graves Disease.)

If the thyroid gland produces insufficient amounts of T4 and T3, then the person may have signs and symptoms associated with underactive thyroid (hypothyroidism) and a slowed metabolism, such as weight gain, dry skin, fatigue, and constipation. Hashimoto thyroiditis is the most common cause of hypothyroidism. Both hyperthyroidism and hypothyroidism can also be caused by thyroiditis, thyroid cancer, and excessive or deficient production of TSH. The effect of these conditions on thyroid hormone production can be detected and monitored by measuring the free T3 or sometimes total T3.