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In brief: How does the thyroid gland work?

Last Update: June 18, 2021; Next update: 2024.

The thyroid gland is a vital endocrine (hormone-producing) gland. It plays a major role in chemical reactions in the body (our metabolism), as well as our growth and development. It helps to regulate many body functions by constantly releasing a certain amount of thyroid hormones into the bloodstream. If the body needs more energy in certain situations – for instance, if it is growing or cold, or during pregnancy – the thyroid gland produces more hormones.

Location and structure

The thyroid gland is found at the front of the neck, under the voice box. It is butterfly-shaped: The two lobes on either side lie against and around the windpipe (trachea), and are connected at the front by a narrow strip of tissue known as the isthmus.

Illustration: Front view of the thyroid gland

Front view of the thyroid gland

The thyroid weighs between 20 and 60 grams on average. It is surrounded by two fibrous capsules. The outer capsule is connected to the voice box muscles and many important blood vessels and nerves. There is loose connective tissue between the inner and the outer capsule, so the thyroid can move and change its position when we swallow.

The thyroid tissue itself consists of a lot of small individual lobules that are each enclosed in a thin layer of connective tissue. These lobules contain a great number of small sacs – called follicles – which store thyroid hormones in the form of little droplets.

The thyroid makes the hormones that the body needs

The thyroid gland produces three hormones:

  • Triiodothyronine, also known as T3
  • Tetraiodothyronine, also called thyroxine or T4
  • Calcitonin

Strictly speaking, only T3 and T4 are proper thyroid hormones. They are made in the follicular epithelial cells of the thyroid.

Iodine is one of the main building blocks of both hormones. Our bodies can’t produce this trace element, so we need to get enough of it in our diet. Iodine is absorbed into our bloodstream from food in our bowel. It is then carried to the thyroid gland, where it is eventually used to make thyroid hormones.

Sometimes our bodies need more thyroid hormones, and sometimes they need less. To make the exact right amount of hormones, the thyroid gland needs the help of another gland: the pituitary gland in the brain. The pituitary gland “tells” the thyroid gland whether to release more or less hormones into the bloodstream. Also, a certain amount of thyroid hormones are attached to special proteins in the blood, known as transport proteins. If the body needs more hormones, T3 and T4 can be released from these proteins in the blood and do their job.

The third hormone produced by the thyroid gland is called calcitonin. Calcitonin is made by C-cells. It is involved in calcium and bone metabolism.

T3 and T4 increase the basal metabolic rate (the amount of energy your body needs to keep up the most basic functions). They make all of cells in the body work harder, so the cells need more energy too. This has the following effects, for example:

  • Body temperature rises
  • Faster pulse and stronger heartbeat
  • Food is used up more quickly because energy stored in the liver and muscles is broken down
  • The brain matures (in children)
  • Growth is promoted (in children).
  • Activation of the nervous system leads to improved concentration and faster reflexes

Hormone imbalances: Overactive and underactive thyroid gland

An overactive thyroid makes too many hormones (hyperthyroidism). An underactive thyroid doesn’t make enough hormones (hypothyroidism). Both of these imbalances can lead to many different symptoms.

The thyroid gland may get bigger too. Sometimes the whole thyroid gland becomes enlarged (diffuse goiter), and sometimes individual lumps called nodules grow in the gland (nodular goiter). A special examination, known as thyroid scintigraphy, can be used to see whether these nodules are producing abnormal amounts of hormones. If they make more hormones than the rest of the thyroid tissue, they are called “hot” nodules. If they make less, they are called “cold” nodules.

In most cases, an enlarged thyroid or nodules aren’t caused by anything serious. They are only rarely cancer. But it’s still important to see a doctor if you notice any changes in your thyroid gland.


  • Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.
  • Lippert H. Lehrbuch Anatomie. Munich: Urban und Fischer; 2017.
  • Menche N. Biologie Anatomie Physiologie. Munich: Urban & Fischer/ Elsevier GmbH; 2016.
  • IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

    Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

    Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

© IQWiG (Institute for Quality and Efficiency in Health Care)
Bookshelf ID: NBK279388


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