The thyroid gland is a vital endocrine (hormone-producing) 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.
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.
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.
The thyroid gland produces three hormones:
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.
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:
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.
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.
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.
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