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hormones
Introduction
The body has a special information system that relies on chemical messengers called
hormones. These organic chemicals are made in ductless endocrine glands that pour
their hormones into the blood. In most cases, the hormones then float to target organs
where they turn biochemical reactions on or off.
The endocrine glands shown here are among the most important glands in the human
body. Hormones …
The major endocrine glands are the pituitary, the thyroid, the parathyroids, the
pancreas, the adrenals, and the ovaries and testes. Their hormones will be discussed in
this article. Hormones are also made in the stomach, the small intestine, and the
kidneys. The pineal and the thymus glands possibly make them, too. The placenta of
pregnant mammals has an endocrine function. It can make pituitary and ovarian
hormones as well as a special one of its own.
Many living things produce hormones. Insects, for example, have hormones that
speed up as well as stop growth at different stages of their life cycle. By maintaining a
proper balance of these hormones, an insect develops in the appropriate way.
Ecdysome is an insect hormone. It influences molting and metamorphosis (see insect).
Plants produce hormones, also. One of the best-known plant hormones is indoleacetic
acid. It promotes rooting and growth.
The Pituitary Controls Other Glands
The pituitary gland (also called the hypophysis) is a small, oval structure under the
brain. It has two parts—the anterior lobe (adenohypophysis) and the posterior lobe
(neurohypophysis). In some animals, the adenohypophysis includes an intermediate
lobe.
The pituitary influences the activity of many other endocrine glands. Most of its
hormones are made in its anterior lobe. Hence, the anterior pituitary is usually called
the “master” gland of the body. Its products are growth hormone (GH), prolactin,
adrenocorticotropic hormone (ACTH), lipotropic hormone (LPH), thyroid-stimulating
hormone (TSH), follicle-stimulating hormone (FSH), interstitial cell-stimulating
hormone (ICSH), and melanocyte-stimulating hormone (MSH). Chemically, the
anterior pituitary hormones are either polypeptides or more complex proteins (see
protein). The posterior lobe does not produce hormones but merely stores two
hormones made in the hypothalamus of the brain. They are vasopressin and oxytocin.
Chemically, they are cyclic polypeptides.
Nearly all the anterior pituitary hormones act on specific tissues. Growth hormone, an
exception, affects the body's overall growth processes. It also aids other hormones in
their work. Prolactin controls the development, growth, and milk production of the
mammary glands. ACTH stimulates the adrenal glands to produce steroid hormones.
TSH prompts the thyroid gland to produce thyroxine. FSH and ICSH together induce
the gonads—ovaries and testes—to make sex hormones. MSH acts on the
melanocytes in the skin to render changes in pigmentation. LPH mobilizes lipid, or
fat, from fatty tissue.
The pituitary must secrete the correct amount of GH for normal early growth. If a
shortage occurs, an infant becomes a dwarf. This disorder, however, can be corrected
by injection of monkey GH or human growth hormone (HGH). By contrast, if the
pituitary produces too much GH in early life, an infant becomes a giant. And if the
gland becomes overactive in adult life, a person develops acromegaly, an enlargement
of the jaw and the extremities. The pituitary must then be removed by surgery.
Vasopressin from the posterior pituitary raises blood pressure by its action on blood
vessels. It is sometimes called antidiuretic hormone because it helps the body retain
water. It prevents the kidneys from producing too much urine, a condition called
diuresis. Without vasopressin, a person develops diabetes insipidus and may excrete
up to 30 liters (about 30 quarts) of urine a day. Oxytocin initiates birth by causing
muscle contraction in the uterus. It also induces milk flow from the mother's breasts
and controls postpartum bleeding (see reproductive system).
The Thyroid Gland Regulates Body Energy
The thyroid gland lies on both sides of the trachea in the neck. Its two lobes connected
by an isthmus resemble the letter H. The average adult thyroid gland weighs about 1
ounce (30 grams). Under the constant direction of TSH, the thyroid converts iodine in
food into thyroxine, an amino acid derivative, and small amounts of chemically
similar triiodothyronine. They regulate both the rate at which food is burned for body
energy and the expression of thyroid hormone-sensitive genes.
Sometimes the thyroid becomes overactive and produces excess thyroxine, a
condition called hyperthyroidism. A hyperthyroid person is nervous, wastes energy,
and becomes irritable easily. The disorder is treated with radioisotopes or by surgery
(see nuclear energy). On the other hand, lack of thyroid hormone, or hypothyroidism,
can arise either from a defective gland or from foods that upset thyroid processes. A
young hypothyroid sufferer becomes a cretin, whose physical and mental growth is
greatly stunted. Thyroxine treatment usually corrects cretinism when caught in time.
The thyroid gland also makes a polypeptide hormone called thyrocalcitonin, which
controls the body's calcium level. It works in conjunction with parathyroid hormone
made by the four tiny parathyroid glands imbedded in the thyroid and vitamin D in
food to help develop and maintain healthy bones.
Basal metabolic rate (BMR) is the medical measurement of thyroid activity. By
regulating BMR the thyroid controls energy output and enables humans and other
animals to adapt to environmental changes, such as hot and cold weather.
The Pancreas and Diabetes
The pancreas plays a vital role by producing two important polypeptide hormones—
insulin and glucagon. They are made in a part of the pancreas called the islets of
Langerhans.
Insulin affects nearly every cell in the body because it is involved in the metabolism
of carbohydrates, fats, and proteins. Lack of insulin causes diabetes mellitus, a
common but potentially fatal ailment (see diabetes). Diabetics have too much glucose,
a sugar, in the blood. Without insulin an untreated diabetic's tissues cannot get the
glucose from the blood for their energy needs. As a result, the diabetic becomes weak.
Other complications occur, such as hyperacidity and dehydration from excessive loss
of water in the urine. Thirst increases. The sufferer can die if the cells fail to replenish
their lost energy and become exhausted. Fortunately insulin injections can correct
diabetes. In milder forms of the disease the pancreas makes insulin but does not
release enough. Certain nonprotein drugs, however, can coax the pancreas to release
life-sustaining amounts of the hormone. Diabetes can easily be discovered by a
glucose-tolerance test. A diabetic will have high blood-sugar levels long after
drinking a glucose solution. Hyperinsulinism is another dangerous pancreatic
disorder. Too much insulin can result in weakness, anxiety, depression, and even
serious convulsions and collapse. Glucose treatment or surgery generally remedy this
problem.
Glucagon, the other pancreatic hormone, tends to raise the blood-sugar level. Both
glucagon and insulin work together to help maintain the normal level of glucose in the
blood so that the body always has a constant and even supply.
The Adrenal Glands: Essential for Life
The vital adrenal glands lie on top of the kidneys. Each gland consists of an outer
cortex and an inner medulla. Each region produces hormones that are chemically
different (see gland).
The adrenal cortex, under the control of ACTH, produces steroid hormones. The
cortical steroids fall into two classes—the glucocorticoids and the mineralocorticoids.
Each in a varying degree affects vital food metabolism and mineral balance. Cortisol,
a glucocorticoid, is used medically to stop inflammation. It is also given to transplant
patients to prevent their bodies from rejecting newly transplanted tissue. Addison's
disease is an adrenal cortex deficiency. It results in low blood pressure, weakness, low
body temperature, and sodium loss. Cortisol and sodium chloride are used to treat it.
An overactive adrenal cortex in women, however, results in hirsutism, an excess of
body hair.
The adrenal medulla produces epinephrine (adrenaline) and norepinephrine
(noradrenaline). Epinephrine triggers the body responses needed when a person
experiences fear, shock, cold, or fatigue. Both hormones raise blood pressure and
heartbeat. Under their influence, glycogen in the body is converted to glucose for
additional energy.
Hormones and Reproduction
The hormones that guide reproductive processes come from the anterior pituitary and
the gonads. During pregnancy, the placenta also makes hormones.
The sex and the form of a developing fetus are affected by events that take place in
the woman's womb. Even though a fetus's sex is determined genetically, the proper
hormones must be available for the fetus to develop the appropriate sex organs. Its
gonads are fairly inactive at birth, but gradual changes take place each day for years
until puberty. Then, changes in the pattern of FSH and ICSH stimulate the gonads to
produce their hormones.
When FSH and ICSH act on the testes, sperm cells develop. Mature sperm usually
form in boys by the age of 16. Testosterone, the major male hormone, plays a part in
sperm formation. It also affects male secondary sexual characteristics and
development of the male accessory sex organs—the prostate gland and the seminal
vesicles. When FSH and ICSH act on an ovary, an egg develops in an ovarian follicle.
Under the influence of FSH and then ICSH, the follicle ripens until it bursts and
releases an egg. The egg then moves down one of the two oviducts for possible
fertilization by a sperm. Meanwhile, the ruptured follicle changes into a corpus
luteum. This tiny structure then produces progesterone, a steroid hormone needed for
maintaining pregnancy. If fertilization does not occur, the corpus luteum degenerates.
Progesterone and estrogen are the female sex hormones. They control the secondary
sexual characteristics, such as body form and voice pitch, as well as development of
the uterus, vagina, and other female accessory sex organs. They are also responsible
for the monthly female “period,” or menstrual cycle. Although irregularities occur,
about once a month in the female body estrogen and then progesterone build up and
maintain the uterine lining in preparation for pregnancy. If the egg released in about
mid-cycle is not fertilized, the uterine lining is sloughed off, menstruation occurs, and
the cycle begins again. This cycle continues until menopause, when the ovaries no
longer function. If fertilization occurs, however, the built-up lining is retained.
Estrogen and progesterone then prepare a place in the uterine wall where the fertilized
egg can lodge and develop into a fetus. The placenta develops and starts to produce
hormones. As the fetus grows, prolactin from the woman's pituitary prepares her
breasts for milk production and flow.
Feedback Control of Hormone Secretion
The pituitary gland coordinates the activity of the endocrine glands to ensure a
hormone balance. A regulatory feedback control system—a push-pull type of
operation—controls pituitary output. For example, the pituitary hormones FSH and
ICSH act on the gonads to produce their respective steroid hormones. When the
concentration of these steroids reaches a certain level in the blood, they act on the
pituitary to cut off any further supply of FSH and ICSH. As soon as the gonadal
hormone level falls, FSH and ICSH output is automatically turned on again. The brain
plays a key part in the feedback operation. It produces chemical compounds that
signal the pituitary to secrete hormones. The gonadal hormones, for example, work by
stimulating the brain to either inhibit or activate production of releasing factors. The
nervous and endocrine systems become integrated through this process. As a result,
the body is assured of a balance of necessary hormones. Brain-thyroid and brainadrenal feedback systems also exist (see biofeedback).
According to the receptor theory of hormone selectivity, hormone-sensitive tissues
have a trapping mechanism, or receptor, that picks up a needed hormone as blood
flows through the tissue. Receptors exist for steroid, polypeptide, and protein
hormones. Once the receptor traps a hormone from the blood, it initiates the
production of second messengers, including cAMP (cyclic adenosine
monophosphate), and cGMP (cyclic guanosine monophosphate). The second
messengers regulate intracellular and cell-to-cell reactions.
Commercial and Medical Uses of Hormones
Doctors can correct hormone deficiencies by giving their patients the needed
hormones. Hypopituitary patients suffering from dwarfism, for example, are given
HGH. Diabetics receive insulin.
Oral contraception, one method of birth control, combines the use of natural ovarian
hormones with slightly modified synthetic ones. Birth control pills use estrogen and
progesterone to inhibit ovulation and thus prevent pregnancy. (See also birth control.)
Sometimes a couple cannot have children because the woman's ovaries are
malfunctioning or the man's testes cannot produce sperm. These problems may be
corrected with FSH and ICSH from human pituitaries. Human chorionic
gonadotropin, a hormone produced solely by the placenta, has also been used.
Hormone therapy for female infertility, however, has sometimes led to
overstimulation of the ovaries and thus to multiple births. (See also fertility and
infertility; multiple birth.)
Estrogen may be given to postmenopausal women to prevent or treat osteoporosis, a
bone disorder marked by a decrease in bone mass and a higher risk of bone fractures.
Estrogen supplements may also be used to relieve the discomforts of menopause and
to treat ovarian disease and certain other conditions. The application of estrogen is
declining, however, because it has been linked with a number of serious health risks,
including several forms of cancer and tumors.
By analyzing the chemical structures of natural hormones, scientists are able to
synthesize artificial substitutes to meet a growing medical demand. Steroid hormones
are not as hard to synthesize as are the more complex polypeptide and protein
hormones. However, advances led to the laboratory synthesis of ACTH, insulin,
thyrocalcitonin, oxytocin, and vasopressin. Polypeptide and protein hormones are
made up of amino acids that must be strung together in a particular order. At first
scientists made hormones by building up the components several pieces at a time and
finally linking them in order. Later, the solid-phase method was devised. In this
procedure, the hormone's tail-end amino acid is attached to a resin or other substance
and then the amino-acid sequence is hooked up in reverse order until the head end of
the hormone molecule is reached. In the final step, the completed chain is freed from
the resin matrix. This technique can be automated. Thus a great deal of synthetic
hormone can be made at a fast rate. Recombinant DNA methods have led to great
advances in the manufacture of such proteins as insulin and HGH.
Recombinant DNA also has been used in the food industry. In the late 20th century
scientists found that bovine growth hormone, or bovine somatotropin (BST), injected
into cattle greatly increased milk and meat production. The United States Food and
Drug Administration in 1993 approved the use of bioengineered BST in cattle
intended for human consumption. The decision was endorsed by many medical and
health organizations, but it remained controversial among consumer, environmental,
and animal-rights groups. The concerns of these groups ranged from food safety to the
effects of synthetic hormones on the animals themselves. Use of growth hormones,
whether synthetically or naturally derived, in food animals is banned in Canada and
the European Union. (See also biochemistry; disease, human; drugs; organic
chemistry; steroids.)