Thyroid

How Does the Thyroid Gland Work?
The thyroid gland (Latin:  glandula thyreoidea) is a vitally important hormonal gland that plays an essential role in metabolism, growth and maturation of the human body. It helps to regulate many body functions. The thyroid gland constantly releases a certain amount of hormones into the blood. More hormones are produced when the body needs more energy in certain situations like growth, coldness or during pregnancy.
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. On average the thyroid gland weighs between 20 and 60 grams and 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 many small individual lobules that are enclosed in thin layers of connective tissue. These lobules contain a great number of small vesicles (sacs) – called follicles – that store thyroid hormones in the form of little droplets. 

Hormone production

The thyroid gland produces five hormones:

1. Triiodothyronine, also known as T3
2. Tetraiodothyronine, also called thyroxine or T4
3. Calcitonin
4. T1
5. T2

Thyroid hormones are made in the follicular epithelial cells of the thyroid.

Iodine is one of the main building blocks of both hormones. Our bodies cannot 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 gut. 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 sends a signal (TSH) to the thyroid gland telling it whether to release more or less hormones into the bloodstream. In addition certain amounts 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 vital hormone produced by the thyroid gland is called calcitonin. Calcitonin is made by C-cells. It is involved in calcium and bone metabolism.

T3 increases the basal metabolic rate. T3 makes all of cells in the body work harder so the cells need more energy, which has the following effects on our body:

• 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 (also known as hyperthyroidism) occurs if the thyroid gland makes too many hormones. An underactive thyroid (hypothyroidism) occurs when the gland does not 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 or Gamma scan, 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 are not caused by anything serious. They are only rarely cancer. However, it is still important to see a doctor if you notice any changes in your thyroid gland.

Sources

• Menche N. (ed.) Biologie Anatomie Physiologie. Munich: Urban Fischer/ Elsevier; 2012
• Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.
• Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Heidelberg: Springer; 2011.
• IQWiG (Institute for Quality and Efficiency in Health Care), a German institute, health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.  
  https://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0072572/