501
CHAPTER THIRTEEN
Endocrine System
The thyroid gland produces
calcitonin,
which is usually
not referred to as a thyroid hormone because it is synthesized
by the C cells, distinct from the gland’s follicles. Calcitonin
plays a role in the control of blood calcium and phosphate ion
concentrations. It helps lower concentrations of calcium and
phosphate ions by decreasing the rate at which they leave the
bones and enter extracellular fl uids by inhibiting the bone-
destroying activity of osteoclasts. At the same time, calcitonin
increases the rate at which calcium and phosphate ions are
deposited in bone matrix by stimulating activity of osteoblasts
(see chapter 7, p. 201). Calcitonin also increases the excretion
of calcium ions and phosphate ions by the kidneys.
Calcitonin secretion is stimulated by a high blood cal-
cium ion concentration, as may occur following absorption
of calcium ions from a recent meal. Certain hormones also
prompt its secretion, such as gastrin, released from active
digestive organs. Calcitonin helps prevent prolonged eleva-
tion of blood calcium ion concentration after eating.
Research suggests that calcitonin may be most important
during early growth and physiological stress. For example, in
the young, calcitonin stimulates the increase in bone deposi-
tion associated with growth. In females, its actions help pro-
tect bones from resorption during pregnancy and lactation,
when calcium is needed for growth of the fetus and synthe-
sis of breast milk.
Table 13.7
summarizes the actions and sources of con-
trol of the thyroid hormones. From Science to Technology 2.1
(p. 54),
table 13.8
, and
f
gures 13.21, 13.22, and 13.23
dis-
cuss disorders of the thyroid gland.
endocytosis, break down the protein, and release the indi-
vidual thyroid hormones into the bloodstream. When the
thyroid hormone levels in the bloodstream drop below a cer-
tain level, this process occurs more rapidly, returning thy-
roid hormone levels to normal.
Once in the blood, thyroid hormones combine with blood
proteins (alpha globulins) and are transported to body cells.
About a third of T
4
is converted to T
3
in peripheral tissues.
Triiodothyronine is nearly ± ve times more potent, but thyrox-
ine accounts for at least 95% of circulating thyroid hormones.
Thyroxine (T
4
)
OH
O
II
CH
2
NH
2
CHCOOH
II
Triiodothyronine (T
3
)
OH
O
I
CH
2
NH
2
CHCOOH
II
FIGURE 13.20
The hormones thyroxine and triiodothyronine have
very similar molecular structures.
TABLE
13.7
|
Hormones of the Thyroid Gland
Hormone
Action
Source of Control
Thyroxine (T
4
)
Increases rate of energy release from carbohydrates; increases rate of protein synthesis;
accelerates growth; stimulates activity in the nervous system
TSH from the anterior pituitary gland
Triiodothyronine (T
3
)
Same as above, but F
ve times more potent than thyroxine
Same as above
Calcitonin
Lowers blood calcium and phosphate ion concentrations by inhibiting release of calcium
and phosphate ions from bones and by increasing the rate at which calcium and phosphate
ions are deposited in bones; increases excretion of calcium by the kidneys
Elevated blood calcium ion concentration,
digestive hormones
TABLE
13.8
|
Disorders of the Thyroid Gland
Condition
Mechanism/Symptoms
Hyperthyroid
Hyperthyroidism
High metabolic rate, sensitivity to heat, restlessness, hyperactivity, weight loss, protruding eyes, goiter
Graves disease
Autoantibodies (against self) bind TSH receptors on thyroid cell membranes, mimicking action of TSH, overstimulating gland
(hyperthyroidism); exopthalmia (protrusion of the eyes) and goiter
Hypothyroid
Hashimoto disease
Autoantibodies (against self) attack thyroid cells, resulting in hypothyroidism
Hypothyroidism (infantile)
Cretinism—stunted growth, abnormal bone formation, mental retardation, low body temperature, sluggishness
Hypothyroidism (adult)
Myxedema—low metabolic rate, sensitivity to cold, sluggishness, poor appetite, swollen tissues, mental dullness
Simple goiter
DeF
ciency of thyroid hormones due to iodine deF
ciency; because no thyroid hormones inhibit pituitary release of TSH, thyroid is
overstimulated and enlarges but functions below normal (hypothyroidism)
previous page 531 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 533 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off