Regulation and Integration of the Body
humans. In fact, calcitonin does not need to be replaced in pa-
tients whose thyroid gland has been removed.
At pharmacological doses (doses higher than normally found in
the body), calcitonin has a bone-sparing eﬀect and is given thera-
peutically to patients to treat Paget’s disease and sometimes oste-
oporosis (bone diseases described in Chapter 6). Calcitonin targets
the skeleton, where it (1) inhibits osteoclast activity, inhibiting bone
resorption and release of Ca
from the bony matrix, and (2) stim-
uptake and incorporation into bone matrix. In other
animals, calcitonin rapidly (but brieﬂy) reduces blood Ca
The Parathyroid Glands
Indicate the general functions of parathyroid hormone.
Te tiny, yellow-brown
are nearly hid-
den from view in the posterior aspect of the thyroid gland
. Tere are usually four of these glands, but
the number varies from one individual to another—as many
as eight have been reported, and some may be located in other
regions of the neck or even in the thorax.
developmental abnormalities and mental retardation are not re-
versible once they appear.
Te most common hyperthyroid pathology is
. In this autoimmune condition, a person makes abnormal
antibodies directed against thyroid follicular cells. Rather than
marking these cells for destruction as antibodies normally do,
these antibodies paradoxically mimic ±SH and continuously
stimulate ±H release.
±ypical symptoms of Graves’ disease include elevated meta-
bolic rate; sweating; rapid, irregular heartbeat; nervousness; and
weight loss despite adequate food. Eyeballs may protrude (
) if the tissue behind the eyes becomes edematous
and ﬁbrous (Figure 16.11b). ±reatments include surgically re-
moving the thyroid gland or ingesting radioactive iodine (
which destroys the most active thyroid cells.
a polypeptide hormone released by the
of the thyroid gland in response to a rise in
levels, does not have a known physiological role in
The parathyroid glands.
The parathyroid glands are located on the posterior
aspect of the thyroid gland and may be more inconspicuous than depicted.
of parathyroid gland tissue (160