Encyclopedia of Health and Medicine

L–O
luteinizing hormone (LH) A peptide HORMONE
the anterior lobe of the PITUITARY GLAND produces
that stimulates hormonal activity related to reproduction.
In men, LH stimulates the development
and function of interstitial cells in the TESTES that
synthesize and release TESTOSTERONE, the primary
male sex hormone. In menstruating women, a
mid–menstrual cycle surge of LH stimulates OVU-
LATION (the maturation and release of an ovum, or
egg). In a woman who is pregnant, the PLACENTA
also produces LH. The HYPOTHALAMUS’s release of
GONADOTROPIN-RELEASING HORMONE (GNRH) stimulates
the pituitary gland to secrete LH. Rising levels
of the sex hormones (ESTROGENS, PROGESTERONE,
and TESTOSTERONE) and INHIBIN cause the hypothalamus
to stop releasing GnHR, ending the pituitary
gland’s secretion of LH.
For further discussion of LH within the context
of the endocrine system’s structure and function,
please see the overview section “The Endocrine
System.”
See also ADRENOCORTICOTROPIN HORMONE (ACTH);
ANABOLIC STEROIDS AND STEROID PRECURSORS; ANTIDI-
URETIC HORMONE (AH); CHORIONIC GONADOTROPIN; FOL-
LICLE-STIMULATING HORMONE (FSH); GROWTH HORMONE
(GH); MENSTRUATION; OXYTOCIN; PROLACTIN; RELAXIN;
THYROID-STIMULATING HORMONE (TSH).
melatonin A peptide HORMONE that the PINEAL
GLAND secretes, the primary function of which is to
regulate the body’s circadian cycle (pattern of
sleep and wake). The pineal gland synthesizes
melatonin from the amino acid tryptophan. The
OPTIC NERVE appears to convey NERVE messages of
outside light and dark from the RETINA to a section
of the HYPOTHALAMUS called the suprachiasmatic
nucleus (SCN). The SCN sends nerve signals to the
pineal gland, which suspends melatonin synthesis.
Darkness causes the nerve messages from the
optic nerve to stop, which in turn ends the signals
from the SCN. When receiving signals from the
SCN, the pineal gland stops melatonin production.
When the signals from the SCN end, the pineal
gland resumes melatonin production. Researchers
believe melatonin causes sleepiness by slowing cell
METABOLISM. Other endocrine processes, such as
the adrenocorticosteroid hormonal cascade that
regulates CORTISOL levels, also slow in conjunction
with the circadian cycle, though researchers are
uncertain about how these processes may be
interrelated.
For further discussion of melatonin within the
context of the endocrine system’s structure and
function please see the overview section “The
Endocrine System.”
See also MELATONIN SUPPLEMENT; STRESS RESPONSE
HORMONAL CASCADE.
multiple endocrine neoplasia (MEN) An inherited
genetic disorder in which numerous tumors
form in various endocrine glands. Several GENE
mutations are responsible for the errant growth of
glandular tissue, which may take the form of
tumors or hypertrophy (overgrowth). Oversecretion
of the affected gland’s hormones then occurs.
The three forms of MEN are
• MEN-1, in which adenomas develop in the
PITUITARY GLAND, PARATHYROID GLANDS, and ISLETS
OF LANGERHANS in the PANCREAS
• MEN-2a, characterized by HYPERPARATHYROIDISM,
PHEOCHROMOCYTOMA, and medullary THYROID
CANCER
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