What is the thyroid gland?
The thyroid gland is a reddish-brown colored organ in the neck region. It is often described as butterfly-shaped. This shape results from the two lateral lobes (lobus dexter and lobus sinister), which are usually of slightly different sizes.
The two lateral lobes are connected by a transverse tissue bridge, the isthmus. There may also be a lobe extending from the isthmus, the lobus pyramidalis. The thyroid gland weighs 18 to 30 grams in adults.
Outer capsule and organ capsule
The thyroid gland is surrounded by two capsules, an outer capsule (also known as the external or surgical capsule) and an inner capsule (also known as the internal or organ capsule). Between the two capsules are the larger blood vessels and the four parathyroid glands at the back of the gland. The organ capsule merges into connective tissue ducts that divide the glandular tissue (parenchyma) into individual lobules.
Thyroid lobules (lobules)
The C cells are located between the follicles. These are also called parafollicular cells. They produce the hormone calcitonin and release it into the blood.
Hormonal control circuit
The formation and release of thyroid hormones is subject to a regulatory circuit:
In the so-called hypothalamus, a section of the diencephalon, the hormone TRH (thyrotropin releasing hormone) is formed and released when the level of thyroid hormones (T3, T4) in the blood is too low. TRH stimulates the release of TSH (thyroid stimulating hormone) in the pituitary gland (hypophysis).
TSH leads to increased production of T3 and T4 in the thyroid gland and to their release from their intermediate stores (follicles) into the blood. In this way, they reach all regions of the body, including the diencephalon and the pituitary gland. An increased T3 and T4 level in the blood inhibits the release of TRH and TSH there, which reduces the hormone production of the thyroid gland (negative feedback).
What is the function of the thyroid gland?
The thyroid gland produces the following hormones:
- Triiodothyronine (T3)
- Tetraiodothyronine (thyroxine or T4)
- Calcitonin (calcitonin)
Effect of T3 and T4
The hormones T3 and T4 have several functions:
They increase the basal metabolic rate by increasing cardiac work, body temperature and the breakdown of fats and glycogen (storage form of carbohydrates in the body).
T3 and T4 also promote growth and brain maturation. Length growth and intellectual development in particular depend crucially on the presence of the right amount of thyroid hormones.
In detail, thyroid hormones have the following effects. They promote:
- the absorption of glucose
- carbohydrate metabolism
- oxygen consumption
- heat production
- the breakdown of cholesterol
- the development of the central nervous system, the genital organs and the bone skeleton
- muscle function
- heart rate and blood pressure
At the same time they inhibit
- the formation of energy-rich phosphates
- the storage of carbohydrates
- the formation of proteins
- the utilization of energy
Effect of calcitonin
Why do we need iodine?
The trace element iodine is particularly important for the physiological function of the thyroid gland. T3 and T4 are both formed by the accumulation of iodine molecules.
The daily iodine requirement of an adult is 180 to 200 micrograms and must be covered by food. The trace element is contained in tiny amounts in all foods. It is only found in larger quantities in products from the sea, for example in sea fish such as haddock, saithe, plaice and cod, as well as in algae.
Where is the thyroid gland located?
The thyroid gland is located in the neck region. It lies behind the neck muscles (the paired sternohyoid muscle and the paired sternothyroid muscle) and in front of the windpipe (trachea), the front and sides of which it surrounds.
The isthmus, which connects the two thyroid lobes, is located at the level of the second to third tracheal cartilage (horseshoe-shaped cartilage rods that give the trachea stability).
The two thyroid lobes extend upwards to the lower edge of the larynx and downwards to the upper thoracic aperture (upper thoracic aperture).
It is closely located to the windpipe (trachea), oesophagus and the common carotid artery (arteria carotis communis). The vocal nerve (nervus recurrens) also runs in the immediate vicinity of the thyroid gland.
What problems can the thyroid gland cause?
Common diseases are hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid).
In the case of hyperthyroidism, the gland produces too many thyroid hormones. This leads to weight loss due to an abnormally high basal metabolic rate, an increase in body temperature, increased cardiac activity, insomnia and inner restlessness, mental instability, hand tremors and diarrhea. Hyperthyroidism is usually caused by an autoimmune disease.
In hypothyroidism, there is a deficiency of thyroid hormones. The result is a low basal metabolic rate, which manifests itself in weight gain, constipation and sensitivity to cold. Other symptoms include doughy thickening and swelling of the skin (myxedema), mental slowness and fatigue, shaggy and dry hair as well as libido and potency disorders. Hypothyroidism can be congenital or acquired.
Various types of inflammatory diseases of the thyroid gland (thyroiditis) are less common. The best-known form of thyroiditis is Hashimoto’s autoimmune thyroiditis.
Benign tumors and cancers of the thyroid gland also occur.
