The body's internal environment is monitored and regulated by a chemical communication network. Working alongside the nervous system, endocrine glands produce hormones that control and coordinate many bodily functions. These hormones regulate the body’s growth, metabolism, sexual development, emotional responses and mental processes.
The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal, ovaries and testes. The pancreas is also a part of this system; it has a role in hormone production as well as in digestion.
Hormones are powerful chemicals that work by altering the activity of their target cells. The hormone does not initiate a cell’s biochemical reactions, but adjusts the rate at which they occur. Endocrine cells secrete their hormones into the fluid surrounding them. From there, the hormones travel through the bloodstream and affect cells and tissues in distant parts of the body.
Although hormones come into contact with essentially all cells of the body they produce an effect only on certain cells called target cells. These cells have receptors that the hormones recognize and bind to, triggering a response inside the cell. Each hormone can only affect specific target cells that possess the right kind of receptors for that hormone. The mechanism is similar to the way a radio broadcast works - though the signal reaches everyone within range, only those tuned to the right frequency will be able to hear it.
A hormone can have several different target cells, though these do not all react to the hormone in the same way. For example, insulin stimulates liver cells to store glucose but prompts adipose cells to store fatty acids.
Once hormones reach their target cells there are two different mechanisms by which they produce a reaction. The mechanism depends upon whether a hormone is water soluble and thereby unable to cross the cell membrane, or fat-soluble and able to move through the membrane into the cell. Water-soluble hormones act by attaching to a receptor on the cell membrane. This activates a cascade of chemical reaction within the cell that ultimately impacts protein synthesis in the nucleus. In contrast, fat-soluble hormones pass through the cell membrane and move directly into the nucleus to impact protein synthesis there.
Most hormones, such as adrenaline, thyroid stimulating hormone and human growth hormone, are water-soluble and built from amino acids (the building blocks of protein). Most fat-soluble hormones, like the sex hormones and glucosteroids, are made from cholesterol, whereas thyroid hormones, also fat-soluble, are composed of amino acids.
Different factors stimulate the production and release of hormones. Some endocrine glands are stimulated by the presence of certain minerals or nutrients in the blood. For example low blood levels of calcium stimulate the parathyroid glands to release parathyroid hormone, and insulin, made in the pancreas, is released in response to rising glucose levels.
Many endocrine glands respond to hormones produced by other endocrine glands. For example, hormones produced by the hypothalamus stimulates the anterior pituitary gland to produce its hormones. These pituitary hormones then stimulate other glands such as the adrenals and thyroid which, in turn, produce their own hormones.
Hormone stimulation often leads to the rhythmic release of hormones with hormone levels rising and falling in a particular pattern. In a few cases, hormone release occurs in bursts rather than rhythmically. An example of this is the "fight or flight" response, where the medulla of the adrenal gland, releases a surge of epinephrine (adrenaline) when stimulated by nerve fibers from the sympathetic nervous system.
Hormones affect target organs at low concentrations. However the duration of their action is limited from seconds to several hours so blood levels need to be maintained at certain levels tailored to the body's need for that specific hormone.
Many hormones are regulated by a negative feedback mechanism, known as homeostasis. The endocrine system is regulated by this feedback loop in much the same way that a thermostat regulates the temperature in a room by shutting off the heat as the room gets warmer. In the case of the hormones regulated by the pituitary gland, a signal is sent from the hypothalamus to the pituitary gland in the form of a "releasing hormone." This stimulates the pituitary to secrete a "stimulating hormone" into the circulatory system, which then signals the target gland to secrete its hormone. As the level of the target hormone rises in the blood, the hypothalamus and the pituitary gland shut down secretion of the releasing hormone and the stimulating hormone, which, in turn, slows the secretion of hormones by the target gland. This system results in stable blood concentrations of the hormones that are regulated by the pituitary gland.
The blood levels of some hormones vary according to the time of day or even month. Levels of female sex hormones, for instance, follow a monthly cycle regulated by the rhythmic release of gonadotropin-releasing (GnRH) hormone from the hypothalamus. GnRH stimulates the cyclical release of two hormones from the pituitary gland, follicle stimulating hormone, which causes egg follicles to develop, and luteinizing hormone, which triggers an egg to release. Growth hormone, cortisol from the adrenal gland and melatonin from the pineal gland all follow diurnal (daily) cycles. Growth hormone and melatonin are highest at night, while cortisol peaks in the morning. Diurnal hormone rhythms are linked with sleep/wake, or light/.dark cycles.
The glands of the entire endocrine systems work together. If one or more glands are not functioning properly, other glands will overwork to try to keep the body on an even keel. This is also true when malfunctioning or absent glands are being improperly treated. This explains why those with hypothyroidism often develop adrenal insufficiency, low sex hormones and blood glucose issues.
When treating malfunctions of the endocrine system, it's well worth keeping these interrelationships and complexities in mind. Individual bodies start with uniquely functioning (or malfunctioning) endocrine system components. They may then encounter unique life circumstances that alter them, it is good to keep in mind that when treating individuals, nothing is black and white and one size does not fit all. One must always start with the basic when treating the endocrine system. This journey takes time and patience.