Hormones Health Article

Hormones

Biochemical agents that transmit messages between components of living organisms.

Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream, where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis. Hormones do not fall into any one chemical category, but most are either protein molecules or steroid molecules. These biological managers keep the body systems functioning over the long term and help maintain health. The study of hormones is called endocrinology.

Hypothalamus

Most hormones are released into the bloodstream by a single gland. Testosterone is an exception, because it is secreted by both the adrenal glands and by the testes. The major site that keeps track of hormone levels is the hypothalamus. A number of hormones are secreted by the hypothalamus, and they stimulate or inhibit the secretion of hormones at other sites. When the hypothalamus detects high levels of a hormone, it reacts to inhibit further production. When low levels of a hormone are detected, the hypothalamus reacts to stimulate hormone production or secretion. The body handles the hormone estrogen differently. Each month, the Graafian follicle in the ovary releases increasing amounts of estrogen into the bloodstream as the egg develops. When estrogen levels rise to a certain point, the pituitary gland secretes luteinizing hormone (LH) which triggers the egg's release into the oviduct.

The major hormones secreted by the hypothalamus are corticotropin releasing hormone (CRH), thyrotropin releasing hormone (TRH), follicle stimulating hormone releasing hormone (FSHRH), luteinizing hormone releasing hormone (LHRH), and growth hormone releasing hormone (GHRH). CRH targets the adrenal glands. It triggers the adrenals to release adrenocorticotropic hormone (ACTH). ACTH functions to synthesize and release corticosteroids. TRH targets the thyroid where it functions to synthesize and release the thyroid hormones T3 and T4. FSH targets the ovaries and the testes where it enables the maturation of the ovum and of spermatozoa. LHRH also targets the ovaries and the testes, helping to promote ovulation and increase progesterone synthesis and release. GHRH targets the anterior pituitary to release growth hormone to most body tissues, increase protein synthesis, and increase blood glucose.

The hypothalamus also secretes other important hormones such as prolactin inhibiting hormone (PIH), prolactin releasing hormone (PRH), and melanocyte inhibiting hormone (MIH). PIH targets the anterior pituitary to inhibit milk production at the mammary gland, and PRH has the opposite effect. MIH targets skin pigment cells (melanocytes) to regulate pigmentation.

Pituitary gland

The pituitary has long been called the master gland because of the vast extent of its activity. It lies deep in the brain just behind the nose, and is divided into anterior and posterior regions. Both anti-diuretic hormone (ADH) and oxytocin are synthesized in the hypothalamus before moving to the posterior pituitary prior to secretion. ADH targets the collecting tubules of the kidneys, increasing their permeability to and retention of water. Lack of ADH leads to a condition called diabetes insipidus characterized by excessive urination. Oxytocin targets the uterus and the mammary glands in the breasts. Oxytocin also triggers labor contractions prior to birth and functions in the ejection of milk. The drug pitocin is a synthetic form of oxytocin and is used medically to induce labor.

The anterior pituitary (AP) secretes a number of hormones, including growth hormone (GH), ACTH, TSH, prolactin, LH, and FSH. GH controls cellular growth, protein synthesis, and elevation of blood glucose concentration. ACTH controls secretion of some hormones by the adrenal cortex (mainly cortisol). TSH controls thyroid hormone secretion in the thyroid. In males, prolactin enhances testosterone production; in females, it initiates and maintains LH to promote milk secretion from the mammary glands. In females, FSH initiates ova development and induces ovarian estrogen secretion. In males, FSH stimulates sperm production in the testes. LH stimulates ovulation and formation of the corpus luteum, which produces progesteronein females, whereas LH stimulates interstitial cells in males to produce testosterone.