Encyclopedia of Health and Medicine

152 The Endocrine System
• MEN-2b, characterized by pheochromocytoma,
neuromas in the mucous membranes of the
MOUTH and eyes, and medullary thyroid cancer
The gene MUTATION determines the type of
MEN. Not all manifestations occur within each
type. Because researchers only identified the gene
mutations responsible for MEN in the late 1990s,
people who have this autosomal dominant disorder
may not realize it runs in their families. Symptoms
depend on the endocrine glands affected and
the extent of the tumors or hypertrophy. The
endocrinologist may suspect MEN based on the
patterns of symptoms though GENETIC TESTING is
necessary to confirm the mutation. Treatment is
surgery to remove the tumors whenever possible,
with follow-up CHEMOTHERAPY or RADIATION THERAPY.
Doctors typically recommend prophylactic thyroidectomy
for people who carry the gene mutation
for MEN-2a or MEN-2b, to head off thyroid
cancer as medullary thyroid cancer can be aggressive
and is nearly certain to develop. People who
have MEN require ongoing medical observation
and treatment for hormonal deficiencies that
result from therapies for the MEN.
See also GENETIC DISORDERS; INHERITANCE PATTERNS;
POLYGLANDULAR DEFICIENCY SYNDROME; ZOLLINGER-
ELLISON SYNDROME.
norepinephrine A peptide substance the adrenal
medulla of the ADRENAL GLANDS and the synaptic
vesicles in the NERVE endings produce. Norepinephrine
functions in the body as a HORMONE,
when synthesized by the adrenal medulla, and as
a NEUROTRANSMITTER when synthesized by BRAIN
structures or nerve cells. Norepinephrine is also a
DOPAMINE precursor (substance the body uses as
the basis for dopamine synthesis). Among the hormones
activated in the STRESS RESPONSE HORMONAL
CASCADE, norepinephrine acts on the BLOOD vessels
to cause them to constrict (vasoconstriction),
helping to raise BLOOD PRESSURE and centralize
blood flow. It also facilitates GLYCOGEN conversion
to GLUCOSE and lipid METABOLISM, activities related
to glucose balance. The HYPOTHALAMUS directs the
adrenal medulla, via the neurotransmitter acetylcholine,
to release norepinephrine. As a neurotransmitter
in the brain, norepinephrine appears
to play a role in mood and emotion. Norepinephrine
is also available as a pharmaceutical DRUG,
used primarily to raise blood pressure in severe
HYPOTENSION (low blood pressure) resulting from
neurologic causes.
For further discussion of norepinephrine within
the context of the endocrine system’s structure
and function please see the overview section, “The
Endocrine System.”
See also EPINEPHRINE; SHOCK.
oxytocin A peptide HORMONE the HYPOTHALAMUS
synthesizes (produces) and the posterior lobe of
the PITUITARY GLAND stores and releases. Oxytocin
influences sexual arousal in men and women. In
women, oxytocin stimulates uterine contractions
during CHILDBIRTH and the milk letdown REFLEX
during BREASTFEEDING. Oxytocin may have additional
functions in men, including a role in SPERM
production. Obstetricians may administer oxytocin
as a pharmaceutical DRUG to stimulate uterine contractions
to induce labor or to strengthen contractions
during childbirth.
For further discussion of oxytocin within the
context of the endocrine system’s structure and
function, please see the overview section “The
Endocrine System.”
See also ADRENOCORTICOTROPIN HORMONE (ACTH);
ANTIDIURETIC HORMONE (ADH); FOLLICLE-STIMULATING
HORMONE (FSH); GROWTH HORMONE (GH); LUTEINIZING
HORMONE (LH); PROLACTIN; THYROID-STIMULATING HOR-
MONE (TSH).