Encyclopedia of Health and Medicine

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 thyroid
gland and the parathyroid glands are physically
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 independently
and cannot remain if it is necessary to
remove the thyroid gland. Absence of all four
parathyroid glands requires lifelong HORMONE THER-
APY with PARATHYROID HORMONE supplement.
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 consequence
of incomplete migration when the structures
separate during fetal development. In the
EMBRYO the top two parathyroid glands arise from
the same tissue as the thyroid gland and the bottom
two from the same tissue as the THYMUS.
Because each parathyroid gland has its own substantial
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 calcium
balance in the body. Calcium is essential for
BONE DENSITY and STRENGTH as well as the conduction
of NERVE impulses and MUSCLE contractions.
The parathyroid glands continuously monitor the
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level of calcium in the BLOOD circulation as blood
flows through them. Parathyroid hormone
increases the amount of calcium in the blood circulation
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 (undersecretion
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 person
is born without parathyroid glands, a CONGENI-
TAL ANOMALY with 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
BLOOD circulation. Parathyroid hormone causes the
bones to release calcium into the blood to meet
the body’s needs. Calcium is essential for the conduction
of impulses among nerves and for MUSCLE
contraction. Calcium helps maintain normal HEART
RATE and rhythm.
Parathyroid hormone functions in dynamic balance
with CALCITONIN, a hormone the THYROID
GLAND produces that lowers blood calcium levels
by stimulating the bones to absorb more calcium.
Parathyroid hormone also enhances the activation