Triiodothyronine: Function & Diseases

Triiodothyronine, also known as T3, is an important hormone produced in the thyroid gland. Together with T4, another thyroid hormone, it plays a major role in many metabolic processes in the human body.

What is triiodothyronine?

Infographic on the anatomy and location of the thyroid gland, as well as the symptoms of hyperthyroidism and hypothyroidism. Click image to enlarge. The hormones triiodothyronine (T3) and thyroxine (T4) are called thyroid hormones. They are of great importance in growth processes and in energy metabolism. Both hormones are very similar and differ only by one iodine atom. Triiodothyronine has three iodine atoms and is therefore also called T3. Thyroxine, also called T4, is accordingly a molecule that has four iodine atoms attached to it.

Production, formation, and manufacture

Both thyroid hormones are produced in special cells of the thyroid gland from the amino acid tyrosine. For one molecule of T3, one to two iodine atoms are attached to every two tyrosine molecules. The thyroid gland therefore requires iodine for production. It receives this in the form of iodide from the blood. Iodine (also spelled iodine) is essential for the human body, which means that it cannot produce iodine itself and is dependent on the supply from outside. The daily requirement of iodide / iodine is 0.1- 0.2mg. If this amount is under- or exceeded over a longer period of time, thyroid diseases may occur. The thyroid gland can produce its hormones in stock and store them in its cells. When needed, the required hormone is then released from the cell into the blood. All thyroxine (T4) is produced in the manner previously described. However, T3, or triiodothyronine, is produced only to a small extent in the thyroid gland. Triiodothyronine is predominantly produced from T4 shortly before it reaches its site of action. For this purpose, an iodine atom is split off so that T4 becomes T3. Selenium is necessary for this process. Selenium, along with iodine, is therefore a very important trace element for thyroid hormones. If T4 later becomes T3 anyway, why does the thyroid gland actually produce the two hormones and not directly T3, triiodothyronine? T4 (thyroxine) is a kind of transport and storage form of thyroid hormone. T4 molecules have a half-life of about five to eight days in the blood. This means that if the thyroid gland were to stop producing hormones abruptly, half of all the T4 molecules secreted would still be found in the blood after five to eight days. T3, on the other hand, has a half-life of only about 19 hours. On the other hand, it is much more effective than T4. When the thyroid gland produces and releases how much triiodothyronine and thyroxine is decided by the anterior pituitary in cooperation with the hypothalamus. Anterior pituitary and hypothalamus are important control centers in the brain. The anterior pituitary produces the hormone TSH (thyrotropin) depending on the body’s need for thyroid hormones. TSH, in turn, stimulates the thyroid gland to produce hormones, secrete hormones, and grow.

Function, action, and properties

In very general terms, triiodothyronine has a stimulatory effect on a large part of the body’s functions. Thus, T3 has a restorative effect on all tissues of the body. Triiodothyronine is particularly relevant for the development of nerve and bone tissue. Thyroid hormones also stimulate the basal metabolic rate in the body, i.e. they are responsible for ensuring that the small “power plants” in the cells, called mitochondria, do their work. Furthermore, they stimulate carbohydrate metabolism. Thyroid hormones are also needed for digestion, because they stimulate intestinal activity. Likewise, triiodothyronine is relevant for the work of muscles.

Diseases, ailments and disorders

Based on the diverse mode of action, it can be guessed that disorders in the area of thyroid hormones can lead to various complaints. Roughly, a distinction is made between hypothyroidism, which is accompanied by a reduction in thyroid hormones, and hyperthyroidism. In hyperthyroidism, too many thyroid hormones are produced. The cause is usually either in the thyroid gland itself or in the control centers pituitary and hypothalamus. In the case of hypothyroidism, the body’s metabolism is shut down. The consequences are fatigue, an increased need for sleep and lack of drive. Even depression can be caused by a lack of triiodothyronine.Due to the reduced metabolic activity and the storage of carbohydrates, which can no longer be properly metabolized without triiodothyronine, water retention forms. Those affected gain weight and suffer from edema (swelling), especially in the legs. The metabolism in the entire body lies idle and almost all tissues in the body are affected. This also results in cool, scaly and dry skin as well as brittle hair and nails. With hyperthyroidism, on the other hand, the metabolism runs at full speed. The skin is warm and reddened, and those affected sweat profusely even without exertion. They lose body weight and suffer from constant restlessness and insomnia due to the hyperexcitability of muscle and nerve tissue. The constant stimulation of muscle tissue leads to muscle weakness. There may even be heart problems, including atrial fibrillation.