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How does the thyroid gland work?

Created: November 17, 2010 ; Last Update: April 19, 2018 ; Next update: 2021.

The thyroid gland is a vital hormone gland: It plays a major role in the metabolism, growth and development of the human body. It helps to regulate many body functions by constantly releasing a steady 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.

This organ (medical term: glandula thyreoidea) is found at the front of the neck, under the voice box. It is butterfly-shaped: The two side lobes lie against and around the windpipe (trachea), and are connected at the front by a narrow strip of tissue.

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 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 enclosed in thin layers of connective tissue. These lobules contain a great number of small vesicles (sacs) – called follicles – which store thyroid hormones in the form of little droplets.

Hormone production as needed

The thyroid gland produces three hormones:

Strictly speaking, only T3 and T4 are proper thyroid hormones. They are made in what are known as 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. 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 transport proteins in the blood. If the body needs more hormones, T3 and T4 can be released from the 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. They make all of cells in the body work harder, so the cells need more energy too. This has the following effects, for example:

Hormone imbalances: Overactive and underactive thyroid gland

An overactive thyroid (also known as hyperthyroidism) occurs if the thyroid gland makes too many hormones. An underactive thyroid (hypothyroidism) is where the gland doesn’t make enough hormones. Both of these imbalances can lead to a great number of symptoms.

The thyroid gland may grow in size 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.

Sources

Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer; 2011.

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