called
osteoclasts
.
Osteoclasts
“eat” bone mineral and release
the calcium into the blood. By in-
hibiting the activity of osteoclasts,
calcitonin can decrease the levels
of calcium in the blood. However,
the normal physiological role of
calcitonin remains a mystery because calcitonin can be
present in excess or completely absent without causing
any abnormal physiology.
When the thyroid gland does not secrete enough thy-
roid hormone, a condition called
hypothyroidism
results.
If it occurs at birth, it is called
congenital hypothyroid-
ism
; if it occurs later in life, it is referred to as
myx-
edema
. Congenital (or primary) hypothyroidism causes
abnormal development and mental retardation, but it can
be treated with oral doses of thyroxine; by law, newborns
must be screened for proper thyroid function. Myxedema
(secondary hypothyroidism) is characterized by puffiness
in the face, slow heart rate, sensitivity to hot and cold, and
muscle weakness. Primary hypothyroidism is caused by de-
fects in the follicular cells of the thyroid, while secondary
hypothyroidism can be caused by damage or defects in the
hypothalamus (diminished TRH) or the anterior pituitary
lobe (diminished TSH).
When the thyroid gland secretes too much thyroxine,
hyperthyroidism
occurs. In the most common form of
this condition, called
Graves disease
, the immune system
produces antibodies that bind to the TSH receptor, mimic
TSH, and overstimulate the thyroid. Patients with Graves
disease may have an enlarged thyroid called a goiter (Fig-
ure 9.9a) and/or peculiar puffiness in the eyes called
ex-
ophthalmos
(Figure 9.9b). Graves disease can be treat-
ed with drugs that block the synthesis of thyroid hormone,
by destroying thyroid tissue with radiation, or by surgically
removing thyroid tissue.
Parathyroid Glands Regulate
Calcium Levels
The
parathyroid glands
are small
glands embedded in the posterior
side of the thyroid gland (Figure
9.10).
The
parathyroid
glands
secrete a protein hormone called
parathyroid
hormone
(PTH)
,
which acts on bone cells (osteo-
clasts) to release calcium and on
kidney cells to reduce calcium ex-
cretion and to release
calcitriol
.
osteoclasts
A type of bone cell
that resorbs, or
"eats," bone mineral
and releases its cal-
cium into the blood.
calcitriol
The active
form of vitamin D,
which is secreted by
the kidneys and stimu-
lates the intestine to
absorb calcium from
food into the blood,
thereby increasing
blood calcium levels.
G raves d ise a se • Figure 9.9
a.
Patients with Graves disease (hyperthyroidism) may
have an enlarged thyroid called a goiter.
b. A puffiness in the eyes
called exophthalmos is another
symptom of Graves disease.
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264 CHAPTER 9
The Endocrine System
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