Hidden by the skin in the frontal area of the neck and wrapped around three sides of the windpipe, just below the “Adams apple”, lies a butterfly-shaped gland called the thyroid. The thyroid is about 5 cm long and has two lobes on either side of the windpipe, which are connected by a strip of thyroid tissue – known as an isthmus – that gives it the typical butterfly shape. The thyroid is one of a number of hormone producing glands that form part of the endocrine system.
This small gland produces hormones that play a crucial role in the functioning of the body.
What the thyroid does: The main functions of the thyroid are to produce and secrete hormones that control metabolism, growth and development. Thyroid hormones play a role in every organ and system in the body, such as:
- The hormones that are secreted by the thyroid interact with all the other hormones in the body, for example insulin, cortisol, estrogen and testosterone. These hormones are all interrelated and in constant communication.
- Metabolism: The thyroid produces the master metabolism hormones and play a vital role in regulating the metabolic rate (the rate at which the body burns calories) of every cell in the body in order to perform its function. In this respect, the thyroid hormones act similar to the conductor of an orchestra, as they influence metabolism by determining the rate at which cells are produced, energy is used and nutrients are absorbed in the body.
- Growth: Thyroid hormones are, for example, essential for normal growth in children and are intertwined with the growth-promoting effect of growth hormones. They signal the production of virtually all growth factors in the body, such as skeletal tissue growth, the development of red blood cells, the nerve growth factor and the epidermal growth factor. Thyroid deficiency can result in growth retardation in children.
- Development: The thyroid hormones play a vital role in the development of the human body, for example, the development of the fetal and neo-natal brain depends on normal levels of thyroid hormones.
- The thyroid hormones have a huge bearing on energy levels and maintenance of normal body temperature.
- The thyroid hormones are partly responsible for the healthy functioning of the organs in the body, for example they play a vital role in regulating the heart rate and blood volume.
- The hormone calcitonin is associated with the regulation of calcium levels in the body, not only for bone growth and development, but also by slowing down the rate at which bone is broken down.
- The thyroid hormones regulate triglyceride and cholesterol metabolism in the liver.
- The thyroid hormones are involved in the production of the hormone prolactin, which is responsible for milk production in pregnant women.
How the thyroid works: The cells in the thyroid gland absorbs iodine – which is found in a variety of foods – and converts the iodine in combination with the amino acid tyrosine into the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These two hormones are released into the blood stream and transported to every cell in the body, where they play a vital role in controlling metabolism.
The thyroid’s hormone secretion consists mostly of T4, which is the inactive form. T4 is mainly converted in the liver, but also in the kidneys and muscles, to T3, which is the active form.
The thyroid also produces the hormone calcitonin, which slows down the rate at which skeletal bone are broken down.
The amount of thyroid hormones released into the blood stream is controlled by the pituitary gland, which secretes more or less of a thyroid stimulating hormone (TSH) in order to keep thyroid hormones at “normal” levels in the blood stream. In its turn the pituitary gland receives its instructions – at what level of production the thyroid should be set – from the hypothalamus gland in the brain, in the form of TSH Releasing Hormone (TRH). The hypothalamus and the pituitary are in constant communication to ensure that thyroid hormone levels are neither too high nor too low.
When released into the blood stream, the thyroid hormones travel to every cell in the body to regulate the speed at which the cells run their metabolism. Inside the nucleus of the cell, hormones interact with the DNA and plays a role in modulating gene expression. In fact, T3 engages thyroid hormone receptors in the nucleus of cells, which together bind to DNA, leading to transcription of thyroid responsive genes that have multiple actions in different cells, affecting cell metabolism.
Maintaining the level of thyroid hormones in the blood stream is a vital function, as both undersecretion and overproduction of thyroid hormones are associated with disease. As the live does most of the conversion of T4 to T3, liver dysfunction can also affect thyroid hormone levels in the body.
(Iodine is found to some degree in foods such as seaweed, seafood, dairy products, grain products, eggs, some fruit and vegetables, and iodized salt.)
Sources:
Mechanism of action and physiologic effects of thyroid hormones. Published in Hypertexts for Biomedical Sciences, by Vivo Pathophysiology, Colorado State University. (www.vivo.colostate.edu)
How your thyroid works. Published online and last updated 24 October 2018. EndocrineWeb. (www.endocrineweb.com)
Thyroid gland: Overview. Published online and last updated 6 July 2018. EndocrineWeb. (www.endocrineweb.com)
Picture of the thyroid. Published online and last reviewed 13 March 2017. WebMD. (www.webmd.com)
Thyroid function and its effects on the body. Published online. The Carlton Clinic. (www.thecarltonclinic.com)
Thyroid gland. Chapter 63, p.633 – p.641, of the handbook Medical Physiology. A systems approach. By Hershell Raff, Michael Levitzky, et al.
How to diagnose and treat thyroid disease. Published online 28 October 2015. Dr. Mercola. (www.mercola.com)
HEALTH INSIGHT
December 2018