Hormones are protein or steroid substances secreted by the ductless (endocrine)
glands to serve as blood-borne "messengers" that regulate cell function elsewhere in the body. They form a communications system in the body and can
bring about extraordinary changes in cell activity.
Hormones exert their effects by altering the rates at which specific cellular
processes proceed. For example,
insulin promotes glucose (sugar)
uptake by cells that
require energy; follicle-stimulating hormone
provokes the ovary into
producing ovum; testosterone enhances the production of spermatozoa in the testes. The
hormones do this by
combining with enzymes, or by enhancing enzyme production in a cell through a chemical effect on the RNA (ribonucleic acid) of the cell. The
result in the body
depends on the target
organ cell of the
hormone and its function.
The nervous and
endocrine systems of the body actually function as a single interrelated system. The central nervous system,
particularly the hypothalamus, plays a crucial role in
controlling hormone secretion; conversely,
hormones greatly alter neural function. No hormone is
secreted at a constant rate, and most hormones are either broken down by the liver or
excreted by the kidneys. The investigation of endocrine
disease therefore depends on
measurements of excretion rates and circulating blood levels; but since many of
these chemical messengers are
bound to protein in the blood, the diagnostic
tests are particularly complicated and expensive.
Disease in the endocrine system is revealed by an
alteration in the expected
effects of the target
organ cells. For example,
growth fails to occur,
ovulation is inadequate, or
myxedema occurs because of
inadequate function of the
thyroid gland. Less frequently the disorder may be that of excessive hormonal secretion, for example, virilization in a woman from a pituitary tumor, or
thyrotoxicosis from a thyroid tumor. The treatment of
hormonal disorder is undertaken by an endocrinologist and
involves the achievement of
balance in the supplements administered; diabetic control by the appropriate daily dosage of insulin is the
clearest example of this.
In therapy, hormones are used as replacement
where the patient is
deficient (as with insulin for the diabetic), as
treatment to overcome disease (for example, cortisone for
arthritis or asthma), and as controlling agents to
divert natural functions (for example, sex hormones as contraceptives). Synthetic compounds, designed to resemble the natural
products but to achieve
enhanced or differing effects, have gradually become available
since the 1930s, and in recent years an
explosive increase in the
knowledge of their benefits---and
their disadvantages---has been achieved.
Considerable improvements in the effectiveness of hormone
treatment and replacement therapy have been made, but the accurate elucidation of each hormone's multiple influences on human physiology is a continuing challenge.