HUMAN BIOLOGY

What are the bodily roles of thyroid

hormone and parathyroid hormone?

• Thyroid hormone (TH) influences the basal metabolic rate,
  growth, and the development and functioning of the
  nervous system.


• Parathyroid hormone (PTH) is the main regulator of calcium
  levels in the blood.

taKe-home message

Health experts estimate that about 16 million people

in the United States have hypothyroidism. Many are

unaware that this thyroid disorder is affecting their

health because the main symptoms, such as fatigue

and weight gain, can have many causes. Research

this topic online. Who is most likely to develop

hypothyroidism? What are treatment options?

THiNk OuTSidE THE BOOk

muscle contraction as well as for the activation of enzymes,
the formation of bone, blood clotting, and other tasks.
The parathyroids secrete more PTH when the blood level
of calcium falls below a set point, and they reduce their
secretions when the calcium level rises. The hormone
calcitonin from the thyroid gland contributes to processes
that remove calcium from the blood.
You may remember that Section 5.1 discussed bone
remodeling, the process in which bone is deposited or
broken down, depending on the level of calcium in the
blood. PTH is the hormone in charge of remodeling, and it
acts on the skeleton and kidneys. When the blood level of
calcium falls below a set point, PTH prompts the bone cells
called osteoclasts to secrete enzymes that digest bone tis-
sue (Figure 15.8B). This process releases calcium ions (and
phosphate) that can be used elsewhere in the body. In the
kidneys, PTH also stimulates the reabsorption of calcium

from the filtrate flowing through nephrons. At the same

time, PTH helps to activate vitamin D. As described earlier,

activated vitamin D is a hormone that improves the absorp-

tion of calcium in the GI tract.

In children who have vitamin D deficiency, too little

calcium and phos phorus are absorbed, so the rapidly grow-

ing bones don’t develop properly. Children who have the

resulting bone dis order, rickets (Figure 15.8C), develop

skeletal abnormalities such as bowed legs.

Calcium is so essential in the body that disorders related

to parathyroid functioning can be quite serious. For exam-

ple, excess PTH (hyperparathyroidism) causes so much cal-

cium to be withdrawn from a person’s bones that the bone

tissue is dangerously weakened. The excess calcium in the

bloodstream may cause kidney stones, and muscles don’t

function normally. The central nervous system’s operations

may be so seriously harmed that the affected person dies.

Figure 15.8 Animated! PTH regulates calcium homeostasis. a The location of the parathyroid glands. The diagram in b shows the
feedback loop that controls the release of PTH. c A child with legs bowed by rickets. (A–B: © Cengage Learning; C: Biophoto Associates/Science Source)

Osteoclasts

release

calcium ions

from bone.

Kidney tubules

increase

calcium ion

reabsorption.

Intestine

increases

calcium ion

absorption.

Decrease in calcium ion

concentration in blood

Parathyroid

glands

PTH

stimulates

Calcium ion

level increases.

parathyroid

glands

pharynx

thyroid

a

b

c

the endOCrIne system 295

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).