Human Physiology, 14th edition (2016)

Regulation of Metabolism 687

normal, while a person who is hyperthyroid may have a basal O 2

consumption up to 50% higher than normal.

A normal level of thyroxine secretion is required for

growth and proper development of the central nervous system

in children. This is why hypothyroidism in children can cause

cretinism. The symptoms of hypothyroidism and hyperthyroid-

ism in adults are compared in chapter 11, table 11.8.

A normal level of thyroxine secretion is required in order

to maintain a balance of anabolism and catabolism. For reasons

that are incompletely understood, both hypothyroidism and

hyperthyroidism cause protein breakdown and muscle wasting.

Growth Hormone

The anterior pituitary secretes growth hormone, also called

somatotropin, in larger amounts than any other of its hormones.

As its name implies, growth hormone stimulates growth in chil-

dren and adolescents. The continued high secretion of growth

food energy that escapes as heat. Because thyroxine stimulates
the production of uncoupling proteins and the rate of cell respi-
ration, the actions of thyroxine increase the production of met-
abolic heat. The heat-producing, or calorigenic ( calor 5  heat),
effects of thyroxine are required for cold adaptation. Recent
evidence suggests that thyroxine has a permissive effect on the
ability of brown adipose tissue to generate heat in response to
sympathetic nerve stimulation. Through this permissive effect,
thyroxine contributes to adaptive thermogenesis as well as to
the basal metabolic rate.
The basal metabolic rate (BMR) has two components—one
that is independent of thyroxine action and one that is regulated
by thyroxine. In this way, thyroxine acts to “set” the BMR. The
BMR can thus be used as an index of thyroid function. Indeed,
such measurements were used clinically to evaluate the condi-
tion of the thyroid prior to the development of direct chemical
determinations of T 4 and T 3 in the blood. A person who is hypo-
thyroid may have a basal O 2 consumption about 30% lower than

Figure 19.14 How epinephrine and glucagon affect metabolism. (1) The hormone binds to its receptor, causing
G-proteins to dissociate. The alpha subunit diffuses through the membrane to activate adenylate cyclase, which catalyzes the
production of cAMP as a second messenger. (3) The cAMP binds to and removes the regulatory subunit of protein kinase, activating
this enzyme. (4) The activation and inactivation of different enzymes by protein kinase promotes glycogenolysis in the liver and lipolysis
in adipose tissue.

α α

α

G-proteins

Cell

membrane

Epinephrine

or glucagon

receptor

Epinephrine

or glucagon Adenylate

cyclase

ATP

Protein kinase

(active)

cAMP + PP

Protein kinase

(inactive)

Regulatory subunit Regulatory subunit

cAMP

Phosphorylation

of enzymes

Glycogenolysis

(liver)

Lipolysis

(adipose tissue)

Activation of

specific enzymes

Inactivation of

specific enzymes

2

1

3

4

β

γ