610
UNIT 3
Regulation and Integration of the Body
16
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
2
1
from the bony matrix, and (2) stim-
ulates Ca
2
1
uptake and incorporation into bone matrix. In other
animals, calcitonin rapidly (but briefly) reduces blood Ca
2
1
levels.
The Parathyroid Glands
Indicate the general functions of parathyroid hormone.
Te tiny, yellow-brown
parathyroid glands
are nearly hid-
den from view in the posterior aspect of the thyroid gland
(Figure 16.12a)
. 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
Graves’ dis-
ease
. 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 (
ex-
ophthalmos
) 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 (
131
I),
which destroys the most active thyroid cells.
Calcitonin
Calcitonin,
a polypeptide hormone released by the
parafol-
licular
, or
C, cells
of the thyroid gland in response to a rise in
blood Ca
2
1
levels, does not have a known physiological role in
(b)
Capillary
Parathyroid
cells
(secrete
parathyroid
hormone)
Oxyphil
cells
Pharynx
(posterior
aspect)
Thyroid
gland
Parathyroid
glands
Trachea
Esophagus
(a)
Figure 16.12
The parathyroid glands.
(a)
The parathyroid glands are located on the posterior
aspect of the thyroid gland and may be more inconspicuous than depicted.
(b)
Photomicrograph
of parathyroid gland tissue (160
3
).
previous page 644 Human Anatomy and Physiology (9th ed ) 2012 read online next page 646 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off