Chapter 6 The Endocrine System • MHR 179Early treatments for goiter involved the simple
addition of small amounts of iodine to the diet.
Later attempts to add iodine to drinking water were
dropped in response to public opposition. The
problem was finally dealt with by the addition of
trace amounts of iodine to common table salt. In
much of the world, the consumption of “iodized
salt” has generally eliminated goiter as a public
health problem.
Figure 6.15An enlarged thyroid gland can result from a
lack of iodine in the diet. Without iodine, the thyroid is unable
to produce thyroid hormones, and continued anterior
pituitary stimulation causes the gland to enlarge.
Calcitonin and
Parathyroid Hormone
Calcium is essential for healthy teeth and normal
skeletal development. This mineral also plays a
significant role in blood clotting, the formation of
nerve impulses, and in muscle contraction. Calcium
levels in the blood are regulated by calcitonin, a
hormone which is produced by the thyroid gland,
and parathyroid hormone(PTH), which is made by
the parathyroid glands. Calcitonin and parathyroid
hormone are antagonistic hormones. They have
opposite effects on blood calcium levels.
High levels of calcium, obtained from dietary
sources, stimulate an increase in calcitonin secretion,
which then increases the rate at which calcium in
blood is deposited into bone tissue of the skeletal
system. This results in a lowering of blood calcium
levels. Calcitonin acts by increasing the rate of
calcium excretion in the kidney.
As illustrated in Figure 6.16 on the following
page, a decrease in blood calcium prompts the
parathyroids to produce more parathyroid
hormone. When blood calcium falls below a
critical threshold level, PTH secretion is stimulated
by a negative feedback loop. Increased PTH
stimulates bone tissue to release calcium into the
blood and increases the rate of re-absorption of
calcium from the kidneys and the duodenum of
the digestive system.In past generations, before they were recognized
as distinct glands, one or more of the parathyroid
glands might have been removed along with the
surgical removal of the thyroid gland. In the absence
of parathyroid glands, the amount of calcium in a
patient’s blood would fall to dangerously low levels.
These patients were then at high risk of developing
tetany, a potentially fatal condition characterized
by uncontrollable, continuous muscular contraction
sustained by the activity of hypersensitive nerve
cells. Due to our current level of understanding of
how these glands function, surgeons no longer use
this procedure.Osteoporosis is the loss of bone mass that results in the
bones becoming brittle and subject to fractures. At least
20 different hormones, growth factors, and vitamins
affect bone formation, along with diet and activity level.
Postmenopausal women are at greatest risk of osteoporosis
because they have less bone mass than men and begin
losing it earlier (starting around age 35). By age 70, the
average woman has lost 30 percent of her bone mass,
and some have lost as much as 50 percent. In men, bone
loss begins around 60 years of age and seldom exceeds
25 percent. Bone mass is acquired primarily during puberty
and adolescence, when high levels of growth hormone,
estrogen, and testosterone stimulate bone formation.
This makes proper diet and exercise critical during
adolesence.