I
ADRENOCORTICOTROPHIC HORMONE
FEEDBACK MECHANISM
The rate at which many glands produce hormones is influenced by other hormones,
especially those secreted by the pituitary gland and the hypothalamus. Ifthe
amount of hormone produced is increased, negative feedback mechanisms act
on the hypothalamus and pituitary so that they produce less of their stimulating
hormones, thus reducing the target gland’s activity. Ifthe amount of hormone
produced is decreased, the feedback weakens, causing increased production of
stimulating hormones.
Anterior pituitarygland
Stimulated bya releasing
factorsecreted by the
hypothalamus, the anterior
pituitarygland produces
adrenocorticotrophic
hormone (ACTH).
ACTH
Released into the
bloodstream bythe
pituitary, ACTH acts
directlyon the cortex
of the adrenal glands.
Adrenal gland
Stimulated bythe ACTH,
the adrenal cortex
produces hydrocortisone
and other corticosteroid
hormones.
Hypothalamus
Influences the pituitaryby
secreting hormones called
releasing factors. The
hypothalamus itself is
influenced byinformation
coming from manyparts
of the body.
. Negative feedback
If the level of
hydrocortisone in the
blood rises, it has a
negative feedback
effect inhibiting ACTH
production bythe
pituitary. This has
the effect of slowing
hydrocortisone
production bythe
adrenal cortex.
response to stimulation by sympathetic
nerves. These nerves are most active
during times of stress.
The release of these hormones into
the circulation produces effects similar
to sympathetic nerve stimulation. The
heart rate and force of contraction of
the heart muscle increase so that more
blood is pumped around the body and
the airways are widened to ease breath-
ing.
The
hormones
constrict
blood
vessels in the intestines, kidneys, and
liver, and widen blood vessels supplying
the
skeletal
muscles.
Consequently,
more blood is supplied to the active
muscles and less to the internal organs.
adrenal hyperplasia, congenital
A rare
genetic disorder,
present from
birth, in which an
enzyme
(a protein
that acts as a catalyst) defect blocks the
production of corticosteroid hormones
(hydrocortisone
and
aldosterone)
from
the adrenal glands.
SYMPTOMS
The enzyme block results in production
of
excessive
amounts
of
androgens
(male sex hormones), which can cause
abnormal genital development in an
affected fetus. In females, these andro-
gens cause enlargement of the clitoris
and some fusion of the outer lips of the
vulva, resulting in genital ambiguity
(see
sex determination).
Some affected
males have an enlarged penis, which
may either be evident at birth or may
develop later.
Other effects of the enzyme defect
include
hypotension
(low blood pres-
sure),
hypoglycaemia
(low blood sugar
levels), weight loss, and dehydration.
Hyperplasia
(enlargement) of the adre-
nal
gland
occurs
due
to
excessive
secretion of the hormone
ACTH,
which
is a result of insufficient production of
hydrocortisone. Excessive skin pigmen-
tation may occur in skin creases and
around the nipples.
In severe cases, congenital adrenal
hyperplasia is apparent soon after birth.
In milder cases, symptoms appear later,
sometimes producing premature puber-
ty in boys and delayed menstruation,
hirsutism
(excessive hairiness), and po-
tential infertility in girls.
DIAGNOSIS AND TREATMENT
A diagnosis is confirmed by measuring
corticosteroid hormones in the blood
and urine.
Ultrasound scanning
may be
carried out to verify that there is no
tumour of the adrenal glands.
Treatment of congenital adrenal hyp-
erplasia is with hormone replacement.
If the treatment is started early, normal
sexual development and fertility gener-
ally follow.
adrenaline
Also called epinephrine, a hormone re-
leased by the adrenal glands in response
to signals from the sympathetic nervous
system, part of the
autonomic nervous
system.
These signals are triggered by
stress, exercise, or emotions such as fear.
Adrenaline increases the speed and
force of the heartbeat. It widens the air-
ways to improve breathing and narrows
blood vessels in the skin and intestine
so that an increased flow of blood
reaches the muscles.
Synthetic adrenaline is sometimes giv-
en by injection as emergency treatment
of
anaphylactic shock
(a severe allergic
reaction) and
cardiac arrest
(a halt in the
heart’s pumping action). Eye-drops con-
taining dipivefrine (a drug that passes
quickly into the tissues and converts to
adrenaline), may be used to treat acute
(closed-angle)
glaucoma.
Regular use can
cause a burning pain and, occasionally,
blurred vision or pigment deposits on
the eye surface. (See also
noradrenaline.)
adrenal tumours
Rare cancerous or noncancerous tum-
ours
in
the
adrenal glands
,
usually
causing excess secretion of hormones.
Tumours of the adrenal cortex may
secrete
aldosterone,
causing
primary
aldosteronism
(also called Conn’s syn-
drome),
or
hydrocortisone,
causing
Cushing’s syndrome.
Tumours of the adrenal medulla may
cause excessive secretion of
adrenaline
(epinephrine) and
noradrenaline
(norep-
inephrine). Two types of tumour affect
the
medulla:
phaeochromocytoma
and
neuroblastoma,
which affects children.
These tumours may cause intermittent
hypertension
(high blood pressure) and
sweating attacks.
Surgical removal of a tumour or an
adrenal gland usually cures noncancer-
ous tumours. Cancerous tumours may
require additional treatment with
radio-
therapy
and/or
chemotherapy.
(See also
adrenal glands
disorders box.)
adrenocorticotrophic hormone
See
ACTH
.
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