P
parathyroid glands Four small endocrine glands,
somewhat orange or yellowish in color, normally
located in two pairs on the back of each lobe of
the THYROID GLAND. Sometimes one or more of the
parathyroid glands is embedded in the tissue of
the thyroid gland, which does not appear to affect
either gland’s ability to function. Though the thy-
roid gland and the parathyroid glands are physi-
cally connected, they are separate structures with
distinct functions. The thyroid gland remains
undisturbed if it is necessary to remove any of the
parathyroid glands. However, the parathyroid
glands have no structure to support them inde-
pendently and cannot remain if it is necessary to
remove the thyroid gland. Absence of all four
parathyroid glands requires lifelong HORMONE THER-
APYwith PARATHYROID HORMONEsupplement.
Occasionally the top two parathyroid glands are
located in the neck well above the thyroid gland
or the bottom two well below in the chest, a con-
sequence of incomplete migration when the struc-
tures separate during fetal development. In the
EMBRYOthe top two parathyroid glands arise from
the same tissue as the thyroid gland and the bot-
tom two from the same tissue as the THYMUS.
Because each parathyroid gland has its own sub-
stantial blood supply, its location is not critical for
proper function.
For the significance of their function the
parathyroid glands are amazingly small, with each
gland ranging in size from about that of a grain of
rice to that of a small pea. The parathyroid glands
produce parathyroid HORMONE (also called
parathormone), which is essential for proper cal-
cium balance in the body. Calcium is essential for
BONE DENSITYand STRENGTHas well as the conduc-
tion of NERVEimpulses and MUSCLEcontractions.
The parathyroid glands continuously monitor the
level of calcium in the BLOODcirculation as blood
flows through them. Parathyroid hormone
increases the amount of calcium in the blood cir-
culation and exists in dynamic balance with CALCI-
TONIN, a hormone the thyroid gland produces that
increases the amount of calcium the bones absorb
from the blood circulation.
Disorders of the parathyroid glands include
HYPERPARATHYROIDISM(oversecretion of parathyroid
hormone) and HYPOPARATHYROIDISM (undersecre-
tion of parathyroid hormone). Either condition
may result from hypertrophy (enlargement) of a
parathyroid gland or from the development of an
ADENOMA, a noncancerous tumor. Cancer of the
parathyroid glands is very rare. Also rarely a per-
son is born without parathyroid glands, a CONGENI-
TAL ANOMALYwith significant health consequences.
For further discussion of the parathyroid glands
within the context of the endocrine system’s
structure and function please see the overview
section “The Endocrine System.”
See also OSTEOPOROSIS; PAGET’S DISEASE OF THE
BONE.parathyroid hormone A peptide HORMONE, also
called parathormone, the PARATHYROID GLANDS
secrete that regulates the level of calcium in the
BLOODcirculation. Parathyroid hormone causes the
bones to release calcium into the blood to meet
the body’s needs. Calcium is essential for the con-
duction of impulses among nerves and for MUSCLE
contraction. Calcium helps maintain normal HEART
RATEand rhythm.
Parathyroid hormone functions in dynamic bal-
ance with CALCITONIN, a hormone the THYROID
GLANDproduces that lowers blood calcium levels
by stimulating the bones to absorb more calcium.
Parathyroid hormone also enhances the activation153