The endocrine system controls or regulates metabolic processes governing energy production, growth, fluid and electrolyte balance, response to stress, and sexual reproduction.
Hormones (chemical transmitters) are released by the endocrine gland into the bloodstream, in which they are carried to tissues that are responsive tot hem (target cells).
The target may be another endocrine gland or an organ or tissue.
Regulation of hormonal control is based on a feedback system.
Usually the feedback control is one of negative function, which means that an increase in one hormone results in a decrease in another substance.
The main endocrine gland controlling the release of other hormones is the pituitary gland (hypophysis).
For this reason it is often called the "master gland".
The anterior lobe of the pituitary secretes tropic (which literally means "turning") hormones that regulate the secretion of hormones from various target organs.
Decreased levels of target cell hormones result in increased secretion of tropic hormones.
As blood concentrations of the target hormones reach normal levels, a negative message is sent to the anterior pituitary to inhibit its production of the tropic hormone.
For example, thyroid-stimulating hormone (TSH) responds to low levels of circulating thyroid hormone (TH).
As blood levels of thyroid hormone reach normal concentrations, a negative feedback message is sent to the anterior pituitary, resulting in a diminished release of thyroid-stimulating hormone.
The pituitary gland is under the influence of the hypothalamus.
Especially in times of stress, the hypothalamus receives messages from the central nervous system that result in the synthesis and secretion of certain hypothalamic chemicals called neurosecretions or releasing factors.
These chemicals are transported by the way of the pituitary portal system to the anterior pituitary, where they stimulate the secretion of tropic hormones.
An example of this is the secretion of corticotropic-releasing factor (CRF) by the hypothalamus, which stimulates the pituitary to secrete adrenocorticotropic hormone (ACTH).
In this instance, the anterior pituitary is the target of the hypothalamus and secondarily effects a response from another target gland, the adrenals, The adrenals in turn secrete glucocorticoids, which have multiple target sites throughout the body.
Not all hormones are dependent on other hormones for their release.
For example, insulin production depends on blood glucose concentrations.
Other hormones not under the control of the pituitary gland are glucagons, parathyroid hormone (PTH), antidiuretic hormone (ADH), and aldosterone.
Because of the interdependent relationship of these glands, a malfunction in one gland produces effects elsewhere in the body.
Endocrine dysfunction may result because of an intrinsic defect in the target gland (primary) or because of the diminished or elevated level of tropic hormones (secondary).
Endocrine problems occur from hypofunction or hyperfunction of the glands.
Primary hypofunction is usually associated with a more profound deficiency of the target gland hormone because little or no hormone is secreted.
In secondary dysfunction, the target glands secrete some of their hormones but in smaller amounts and less rapidly.
Hyperfunction may be the result of an increase in the tropic hormones (secondary) or a hypersecretion of the target glands.
The major hormones that promote physical growth are thyroid hormone, growth hormone and sex hormones.
Insulin can be said to promote growth by its effect on carbohydrate metabolism, whereas cortisol inhibits growth.
Therefore, deficiencies of growth-promoting hormones or an excess of cortisol can cause growth retardation in children.
Endocrine deficiencies can be the result of abnormal secretory function in the glands responsible for their production, the pituitary hormones stimulate their secretion: or the releasing factors from the hypothalamus.
In some instances growth retardation may be the result of increased production of factors that inhibit hormone secretion.