The thyroid gland produces hormones that influence essentially every organ, tissue and cell in the body. The gland is located in the front central area at the base of your neck and has a butterfly shape. Although it is small and is a little more than two inches wide, it is the largest endocrine gland in the human body. The thyroid gland is a soft tissue and when healthy it is smooth and not tender.
The main function of the thyroid gland is to produce, store and release into the blood stream the following hormones:
- thyroxine or T4 that has four iodine atoms
- triiodothyronine or T3 that has three iodine atoms
- diiodotyronine or T2 that has two iodine atoms
- monoiodotyronine or T1 that has single iodine atom
Thyroid hormone production
Two main thyroid hormones T3 and T4 are iodinated derivatives of the amino acid tyrosine, which regulates growth, development and metabolism, the rate at which the body converts food into energy. Thyroid hormones promote protein synthesis in almost all types of body tissue, affect oxygen consumption, increase the rate of absorption of carbohydrates from the small intestine and peripheral utilization of glucose.
To build thyroid hormones the human body needs iodine and tyrosine. Thyroid requires at least 150 micrograms of iodine per day to make thyroid hormone and this amount can normally be obtained from your diet. Sources of dietary iodine include food and food additives (kelp and seaweed, iodinated salt, iodine additives to bread/flour, preservatives, red coloring, therapeutics (amiodarone, vitamins, Lugol’s solution, etc.), topical antiseptics, and contrast dyes, among others. During the digestion, iodine converts to iodide, which attaches to protein molecules and is carried to the thyroid gland through the blood supply.
In the thyroid gland iodide is oxidized to iodine by thyroid peroxidase (TPO) and incorporated into thyroglobulin molecule (Tg).Concentration of iodine inside the cells of the thyroid gland is 20 to 40 times greater than in the blood. In the adults with sufficient iodine intake approximately 15 to 20 mg of iodine is concentrated in the tissue of the thyroid gland that is about 30% of iodine in the body.
The human body gets all essential amino acids from protein containing food such as meat, fish, poultry, eggs and milk. In the digestive system, the proteins convert into amino acids, one of which is tyrosine. Tyrosine enters the blood flow and the thyroid gland extracts it to combine with iodide and oxygen to build thyroid hormones.
The resulting product is either monoiodotyrosine (MIT) with a single iodide molecule or diiodotyrosine (DIT) with two iodide molecules binded to tyrosine. The combination of MITwith DIT gives T3 and combination of DIT with another DIT produces T4. The thyroid gland stores T4and T3 and releases it from the follicular cell into the blood stream when the body requires it.
There is significantly more T4 hormone produced in the thyroid gland than T3. The ratio of T4 to T3 released in the blood is about 20 to 1. T4 hormone also lasts longer in the body and has a biological half-life of approximately one week, as compared to about 12 hours or less for the T3 hormone. In the liver and peripheral tissues, T4 converts to either T3 or reverse T3 (Tr3). About 83% of T3 hormone is produced through this conversion. In another instance, reverse T3, is biologically inactive and blocks the action of T3.
How thyroid hormones work
Once T4 and T3 are secreted into the bloodstream, proteins known as thyroid-binding globulins (TGBs) bind most of these hormones and transport them in the blood. Only a small amount of T4 and T3is in a free or unbound state and works at the cellular level.
When the free thyroid hormones reach the tissues where they are needed they enter the cells. In the cells free T4 converts into T3. This process occurs mainly in the liver as well as in kidneys and other peripheral tissues. In the cells T3 links to special receptor sites and controls chemical and enzymatic reaction, use of oxygen and other nutrients for the energy production. Thyroid hormones speeding up or slowing down the cell’s activities and are responsible for the speed of metabolism in the body. At the same time, T3 is the active form of thyroid hormone whereas T4 is metabolically inactive.
Thyroid hormones affect body temperature, appetite, energy levels, skeletal development, muscle tone, growth and repair, cardiac rate, fluid balance, blood sugar levels, oxygen utilization, bowel function, autonomic nervous system, cholesterol and triglycerides levels, regulation of carbohydrate, protein and fat metabolism. They are responsible for our general health and well being.
The thyroid and peripheral tissues produce only a trace of T1 and T2 thyroid hormones that are chemical precursors of T3 and T4. Little is known about the effects or the purpose of these hormones. Recent studies suggest that T2 may also be a metabolically active hormone that targets mitochondria and affects metabolism.
The last hormone, calcitonin, is involved in the regulation of bone turnover and maintenance of calcium balance. If the calcium levels in blood are high, calcitonin is released to lower these levels by inhibiting uptake of calcium from the gastrointestinal tract and by promoting its storage in the bones. It also suppresses loss of calcium when levels are low. However, calcium levels are controlled in a higher degree by the parathyroid gland and are more dependent on the hormone estrogen and vitamin D.
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