Endocrine glands are ductless glands, that produce hormones. Structurally, the endocrine glands are cells which produce secretions that diffuse directly into the blood stream. Endocrine glands are differentiated from exocrine glands, which consist of a group of cells which pour their secretions into a duct, and from there into another organ such as the intestine.
Endocrine glands produce chemicals called hormones which act on target organs elsewhere in the body. Hormones can be small molecules (eg vasopressin) or large ones (eg insulin). They have varying effects on the body. The major endocrine glands are the pituitary, thyroid, parathyroids, adrenals, pancreas, ovaries and testes. There are several other minor endocrine glands.
The pituitary gland is a tiny organ, about a centimeter in size, located at the base of the brain. It lies within a bony cage, the sella turcica. It is considered the master endocrine gland, as its hormonal secretions control several other endocrine glands. The pituitary consists of two parts, an anterior (front) and a posterior (rear). The anterior portion produces the following hormones:
- growth hormone, GH, which controls the growth of bones
- thryrotrophic hormone (thyroid stimulating hormone, TSH), which controls the secretions of the thyroid gland
- gonadotrophic hormones, which control the sex glands, ie the testes in males, and the ovaries, the uterus and the breasts in females
- adrenocorticotrophic hormone (ACTH), which controls the secretions of the adrenal glands.
The posterior pituitary produces antidiuretic hormone (ADH) (also called vasopressin) and oxytocin.
- ADH helps the kidneys retain urine: in the absence of ADH the patient may produce copious amounts of urine. ADH also contracts the muscles of the arteries, and increases the blood pressure.
- Oxytocin causes contraction of the uterus and production of milk. It may have a role to play in parturition (the process of childbirth).
The pituitary may produce tumours, called adenomas. These tumours are not cancerous, but can exert pressure on the brain, causing headaches, and on the nearby nerve of the eye, causing changes in vision. Adenomas sometimes produce hormones, depending upon the cells from which they arise (usually ACTH or GH producing cells).
- Adenomas arising from ACTH producing cells can produce large quantities of ACTH, resulting in enlargement of the adrenal glands, which this hormone controls. The adrenal responds by an increased production of steriods, resulting in a disease called Cushing's syndrome. In Cushing's syndrome the patient has weight gain, hirsuitism, menstrual changes, diabetes, raised blood pressure, osteoporosis and increased risks of infection. The same picture may be produced by giving high doses of steroids for long periods. The disease is dangerous and untreated has a fatal outcome.
- Adenomas from GH producing cells produce increased amounts of growth hormone. This results in a disease called acromegaly, characterized by overgrowth of all the bones in the body. The face changes, and the patient may notice an increase in the size of his headwear, gloves and shoes. Diabetes is often present, since GH raises blood sugar levels. Death can occur from heart failure, diabetes or from local effects of the tumour.
Treatment of pituitary tumours is by surgery or radiotherapy. Surgery, earlier accompanied by marked side effects, is now less traumatic with the development of microsurgical methods and stereotactic surgery.
The right and left adrenal glands (also called the suprarenal glands) lie just above the kidneys on either side. The outer layer of the gland is the cortex, and the inner core is the medulla. The cortex produces corticosteroids. The medulla produces adrenaline and noradrenaline (these two hormones are not steroids).