The thyroid is an essential hormonal gland that plays a major role in many of the body’s functions.
The thyroid’s function is to regulate the body’s metabolism, which affects how fast or slow things occur in the body, such as hair growth, heartbeat, and food excretion.
The thyroid also controls how fast or slow you burn calories, which in turn affects your weight.
If a person’s thyroid gland is not in control, it can work in extremes, so it is important to try to keep its function in a midrange.
The thyroid (Latin: glandula thyreoidea) is a vitally important hormonal gland that plays a major role in the metabolism, growth and maturation of the human body. It helps to regulate many body functions by constantly releasing a steady amount of hormones into the bloodstream. More hormones are produced when the body needs more energy, like when it is growing or cold, or during pregnancy.
The organ is located in the front part of the neck below the voice box and has the shape of a butterfly: The two side lobes on the right and on the left are connected at the front by a narrow strip of tissue. The side lobes enclose the windpipe and end at the sides and behind the windpipe.
On average, the thyroid weighs between 20 and 60 grams and is surrounded by two fibrous capsules. The outer capsule is connected to the voice box muscles and to many important vessels and nerves. There is loose connective tissue between the inner and the outer capsule, so that the thyroid can move and change its position when we swallow.
The thyroid tissue itself consists of a lot of small individual lobules joined together by thin sheaths of connective tissue. These lobules contain a great number of small vesicles, called follicles, which store thyroid hormones in the form of little droplets.
The thyroid gland produces three hormones:
Triiodothyronine or T3
Tetraiodothyronine, also called thyroxine, or T4
Strictly speaking, only T3 and T4 are proper thyroid hormones. They are produced in what are known as the follicular epithelial cells of the thyroid, with iodine being one of the main components of both hormones.
Illustration: Thyroid gland cells
Thyroid gland cells
This makes iodine an important substance for thyroid metabolism. It is a trace element, which means that the body cannot produce it itself, but needs to take it in with food. In the intestine, iodine is taken from the food and enters the blood stream. After a few intermediary steps it is built into the thyroid hormones in the thyroid gland.
Since the thyroid hormones are vitally important, there is usually always a sufficient amount of T3 and T4 in the body. Some of the hormones are stored in the thyroid as droplets; others are tied to carrier proteins in the blood. When the body needs more hormones, T3 and T4 can be released from the proteins in the blood and take their effect. Moreover, the follicles in the thyroid tissue can supply more hormones.
The third hormone produced in the thyroid gland is calcitonin, which is produced by C-cells. Calcitonin is involved in calcium and bone metabolism.
T3 and T4 increase the basal metabolic rate. All body cells then work harder and therefore need more energy. This means:
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
Brain maturation is promoted (in children)
Growth is promoted (in children)
Activation of the nervous system leads to higher levels of attention and quicker reflexes
There can be a number of consequences if the thyroid produces too few or too many hormones. With an overactive thyroid, also called hyperthyroidism, it produces too many hormones. This speeds up energy metabolism, and the following symptoms can occur:
Hot flashes, sweating
Emotional instability and irritability or fatigue
Insomnia and restlessness
There can be many reasons for an overactive thyroid. It is often caused by an autoimmune disease of the thyroid tissue, called Graves disease (or Basedow’s disease). Sometimes the overactivity is caused by problems in the thyroid axis: The thyroid function is normally regulated by a hormone produced in the pituitary gland (thyroid-stimulating hormone, or TSH for short), which ensures that neither too much nor too little thyroid hormone is produced. Sometimes the thyroid cells do not react to this regulating hormone anymore – this is then called autonomy.
If, on the other hand, too little thyroid hormone is produced, the body functions slow down. This is called underactive thyroid (hypothyroidism) and can either be genetic or develop in the course of life. There can be many different causes for an underactive thyroid. At our latitude (in Germany), one reason might be a lack of iodine. Getting enough iodine through your diet is therefore important for normal thyroid function. A condition called Hashimoto’s thyroiditis, a chronic inflammation of the thyroid, can also lead to underactivity. The use of certain medications can also affect the production of thyroid hormones.
It is particularly important for children and babies to have enough thyroid hormones, because a lack of these hormones at an early stage of life can have severe effects on physical and emotional development. Thyroid underactivity in adults often develops gradually, which is why people might not notice symptoms for a while. Possible symptoms can include:
General loss of energy and power
Difficulties concentrating or mental slowness
Sensitivity to cold
Waxy skin thickening and swelling (myxedema)
Deep, hoarse voice
Brittle, dry hair
Loss of sexual desire or potency problems
Sometimes even depression
Especially in older people these symptoms are often not recognized as a result of an underactive thyroid, but can be confused with normal signs of ageing.
In hypothyroidism the thyroid gland can get bigger and become visible as a “goiter”, also called “struma.” Hyperthyroidism can cause a visible enlargement of the thyroid gland as well.
A goiter can cause a feeling of pressure in the neck or difficulties swallowing. If the thyroid becomes a lot bigger, the tissue expands downward due to the lack of space. This can sometimes lead to a constriction of the windpipe or to difficulties breathing.
There are different types of enlargement of the thyroid. Either the entire gland is enlarged evenly (struma diffusa), or only defined regions are enlarged in forms of a nodular goiter. A special examination, thyroid scintigraphy, is used to measure hormone production in the thyroid.
Increased hormone production in the area of a nodule is called a hot nodule; if hormone production is lower than in the rest of the thyroid tissue, it is called a cold nodule.
Sometimes an enlargement or nodular changes of the thyroid can be signs of a malignant disease of the thyroid tissue. If you notice such a change or are in any doubt, it may be a good idea to see a doctor.
Think of your thyroid as a car engine that sets the pace at which your body operates. An engine produces the required amount of energy for a car to move at a certain speed. In the same way, your thyroid gland manufactures enough thyroid hormone to prompt your cells to perform a function at a certain rate.
Just as a car can’t produce energy without gas, your thyroid needs fuel to produce thyroid hormone. This fuel is iodine. Iodine comes from your diet and is found in iodized table salt, seafood, bread and milk. Your thyroid extracts this necessary ingredient from your bloodstream and uses it to make two kinds of thyroid hormone: thyroxine, also called T4 because it contains four iodine atoms, and triiodothyronine, or T3, which contains three iodine atoms. T3 is made from T4 when one atom is removed, a conversion that occurs mostly outside the thyroid in organs and tissues where T3 is used the most, such as the liver, the kidneys and the brain.
Once T4 is produced, it is stored within the thyroid’s vast number of microscopic follicles. Some T3 is also produced and stored in the thyroid. When your body needs thyroid hormone, it is secreted into your bloodstream in quantities set to meet the metabolic needs of your cells. The hormone easily slips into the cells in need and attaches to special receptors located in the cells’ nuclei.
Your car engine produces energy, but you tell it how fast to go by stepping on the accelerator. The thyroid also needs some direction; it gets this from your pituitary gland, which is located at the base of your brain. No larger than a pea, the pituitary gland is sometimes known as the “master gland” because it controls the functions of the thyroid and the other glands that make up the endocrine system. Your pituitary gland sends messages to your thyroid gland, telling it how much thyroid hormone to make. These messages come in the form of thyroid-stimulating hormone (TSH).
TSH levels in your bloodstream rise or fall depending on whether enough thyroid hormone is produced to meet your body’s needs. Higher levels of TSH prompt the thyroid to produce more thyroid hormone. Conversely, low TSH levels signal the thyroid to slow down production.
The pituitary gland gets its information in several ways. It is able to read and respond directly to the amounts of T4 circulating in the blood, but it also responds to the hypothalamus, which is a section of the brain that releases its own hormone, thyrotropin-releasing hormone (TRH). TRH stimulates TSH production in the pituitary gland. This network of communication between the hypothalamus, the pituitary gland, and the thyroid gland is referred to as the hypothalamic-pituitary-thyroid axis (HPT axis).