In brief: How does the thyroid gland work?

Location and structure of the thyroid gland

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.

Front view of the thyroid gland

The thyroid typically weighs between 20 and 60 grams. 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 consists of many 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.

What hormones does the thyroid make?

The thyroid gland produces three hormones:

• Triiodothyronine (T3)
• Tetraiodothyronine (T4), also called thyroxine
• Calcitonin

Only T3 and T4 are considered proper thyroid hormones. Calcitonin is made by C-cells. It is involved in calcium and bone metabolism.

Iodine is an important substance that is needed to make the thyroid hormones T3 and T4. 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 used to make thyroid hormones.

How do the thyroid hormones work?

The more active T3 and T4 become in the body, the more the basal metabolic rate goes up (the amount of energy your body needs while at rest). They make all of the cells in the body work harder. This has the following effects, for example:

• Body temperature rises
• The heart beat becomes stronger and the pulse faster
• 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 higher levels of attention and quicker reflexes

How is the production of hormones regulated?

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. It is part of the brain and it regulates many of the processes inside our body. One of the things it does is to use the hormone TSH to control the amount of hormones the thyroid gland releases into the bloodstream.

Most of the thyroid hormones in the bloodstream are bound to proteins, which makes them inactive. If the body needs more hormones, T3 and T4 can be released from these proteins in the blood and do their job.

Thyroid problems and diseases

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).

Various tests can be used to diagnose medical conditions affecting the thyroid.

Sources

• Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.

• Lippert H. Lehrbuch Anatomie. München: Urban und Fischer; 2020.

• Menche N. Biologie Anatomie Physiologie. München: Urban und Fischer; 2023.

• Pschyrembel Online. 2024.

• 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.