Endocrine glands are nine specialised organs: three are located in the brain (the pineal gland, pituitary gland and hypothalamus) and six are located in other parts of the body (the thymus, thyroid gland, adrenal glands, pancreas, testicles and ovaries). They produce hormones — unique chemical messengers, ‘information transporters’ that play a key role in keeping our bodies functioning properly. Each of these organs is surrounded by a network of tiny blood vessels — capillaries — that deliver various substances necessary for hormone production. The thyroid gland, for example, secretes two very important hormones — triiodothyronine and thyroxine. They penetrate almost all cells of the body and affect energy metabolism. This, in turn, has a global impact on heart rhythm, breathing rate, body temperature and the coordinated work of the digestive organs. When there is too little of these substances, fatigue and apathy are observed. If, on the contrary, there is too much of them, insomnia develops and irritability occurs. Hormones are synthesised not only in special glands, but also in individual cells of certain organs, such as the stomach and intestines, and thus they also participate in endocrine regulation and are affected by hormones from other parts of the body.

In the human body, hormones are used for two types of signal transmission. The first type is needed for communication between two endocrine glands, when one of them releases a hormone that stimulates the other gland to change its hormone level. This is how the pituitary gland works. The second occurs between a gland and any other organ: for example, when the pancreas secretes insulin, which affects the uptake of glucose by muscle and fat cells. Some types of hormones target only one or a few tissues. Others are not as specific and can bind to their receptors in various cells throughout the body. For example, estrogens secreted by organs of the reproductive system can regulate cells in the uterus, breast, and even bone. Hormones determine the functional state of our bodies. Thus, the balance between male and female sex hormones, will determine the appearance, gender traits such as roundness of mammary glands, presence of body hair, timbre of voice and others. The adrenal glands produce hormones that control the stress response, and the thymus plays a major role in the production of immune system cells. The endocrine system affects the way the whole body works. Sometimes hormone levels are too high or too low: an imbalance can occur at any time, regardless of age, and can cause serious health problems. When hormone levels are within normal limits, they help the body to cope with internal tasks and interact effectively with the environment.