C

calcitonin A peptide HORMONEthe THYROID GLAND
produces that increases the amount of calcium the
bones can accept. Calcitonin functions in synchro-
nization with PARATHYROID HORMONE, which the
PARATHYROID GLANDSsecrete, to maintain calcium
balance in the BLOODand in the body. Calcium is
essential for BONE DENSITYand STRENGTH, as well as
for the conduction of NERVEimpulses in MUSCLEtis-
sue, including that of the HEART. The balance
between calcitonin and parathyroid hormone
maintains a constant level of calcium in the blood
to allow proper nerve conduction for heart func-
tion as well as skeletal muscle function. This bal-
ance may come at the expense of the amount of
calcium in the bones, however, which the body
uses as a “bank” for the storage and withdrawal of
calcium. Calcitonin facilitates calcium “deposit” to
the bones by channeling calcium from the blood
into the bones. It binds with receptor sites on the
surfaces of osteoblasts, the cells within the bones
that produce new BONEtissue.
A form of THYROID CANCER, medullary thyroid
cancer, forms in the parafollicular cells that pro-
duce calcitonin. Excessive levels of calcitonin in
the blood may indicate such a cancer. Disorders of
the parathyroid glands that affect the secretion of
parathyroid hormone may also disrupt the levels
of calcitonin in the blood.
For further discussion of calcitonin within the
context of the endocrine system’s structure and
function, please see the overview section “The
Endocrine System.”
See also OSTEOPOROSIS; THYROXINE (T 4 ); TRI-
IODOTHYRONINE(T 3 ).

cholecystokinin See DIGESTIVE HORMONES.

chorionic gonadotropin A peptide HORMONEof

PREGNANCYthat the fertilized ovum (egg) secretes to

stimulate the corpus luteum to continue producing

PROGESTERONE, which maintains an environment

within the UTERUSto support the implantation of

the EMBRYO. The corpus luteum, a temporary

endocrine structure, develops in an ovarian follicle

after the follicle releases an ovum, marking the

start of OVULATION. The corpus luteum then begins

secreting progesterone, which causes the lining of

the uterus to thicken and its BLOODsupply to enrich

in preparation for pregnancy. Without chorionic

gonadotropin the corpus luteum deteriorates and

the uterine lining sloughs away (MENSTRUATION).

About eight weeks into pregnancy the PLACENTA

forms and takes over chorionic gonadotropin pro-

duction, which continues until birth. Pregnancy

tests measure the amount of chorionic

gonadotropin in the URINEor the blood. Fertility

specialists use injections of chorionic gonadotropin

(called human chorionic gonadotropin, or hCG, in

its pharmaceutical form) to stimulate ovulation in

women and TESTOSTERONEproduction, to encourage

SPERMproduction, in men.

For further discussion of chorionic gonado-

tropin within the context of the endocrine sys-

tem’s structure and function, please see the

overview section “The Endocrine System.”

See also ANABOLIC STEROIDS AND STEROID PRECUR-

SORS; CONCEPTION; ESTROGENS; FERTILITY; OXYTOCIN;

PROLACTIN; RELAXIN.

corticotropin-releasing hormone (CRH) A pep-

tide HORMONEthe HYPOTHALAMUSsynthesizes and

releases in response to endocrine and neurologic

signals that report the status of the body’s vital

functions. CRH is one of the hormones in the

115