HUMAN BIOLOGY

the thyroid and parathyroid glands

n    Hormones from the thyroid gland are required for normal

growth and metabolism. The thyroid and parathyroid glands

work together to regulate calcium levels in the blood.

n    Links to Bone growth and remodeling 5.1, Autoimmune

disorders 9.10, Major dietary minerals 11.13

Thyroid hormones affect metabolism and growth

The thyroid gland is located at the base of the neck in front

of the trachea, or windpipe (Figure 15.6). The main hor-

mones it produces, thyroxine (T 4 ) and triiodothyronine (T 3 ),

are known jointly as TH (thyroid hormone). TH affects cells

throughout the body. It is largely responsible for setting

a person’s basal metabolic rate

(Section 11.14). It also enhances

the production of GH, and in this

way has a major influence on

growth. Adequate TH is essential

in order for the central nervous

system of a fetus to develop prop-

erly. Optimal functioning of an

adult’s CNS depends on it as well.

The thyroid also makes the

hormone calcitonin, which helps

lower the level of calcium (and of

phosphate) in blood in response

to homeostatic feedback.

TH cannot be formed with-

out iodide, a form of iodine.

Iodine-deficient diets cause one

or both lobes of the thyroid gland

to enlarge (Figure 15.7). The

enlargement, a simple goiter, occurs after low blood levels

of TH set in motion a negative feedback loop that causes the

anterior pituitary to secrete TSH (the thyroid-stimulating

hormone thyrotropin). The thyroid attempts to make TH

but cannot do so, which leads to continued secretion of

TSH, and so on, in a sustained abnormal feedback loop.

Simple goiter is no longer common in places where people

use iodized salt.

Hypothyroidism is the clinical name for low blood levels

of TH. Metabolism slows in affected adults, so they tend to

gain weight, feel sluggish physically and mentally, and find

it difficult to tolerate cold temperatures.

Graves’ disease and some other conditions are due to

hyperthyroidism, in which metabolic activity “revs up” due

to excess of TH in the blood.

Symptoms include elevated

heart rate and blood pressure

and unusually heavy sweating.

Some cases are autoimmune

disorders, in which antibodies

wrongly stimulate thyroid cells.

In other cases the cause can

be traced to inflammation or

a tumor in the thyroid gland.

Some people are genetically pre-

disposed to the disorder.

pth from the parathyroids is

the main calcium regulator

Most of us have four parathyroid glands located on the

back of the thyroid gland (Figure 15.8A). These little glands

secrete parathyroid hormone (PTH), the main regulator

of the calcium level in blood. Calcium is important for

Figure 15.7 a diet low in the
micronutrient iodine may
cause a simple goiter.
(Scott Camazine/Science Source)

trachea

(windpipe)

thyroid

gland

thyroid

cartilage

(Adam’s

apple)

blood

vessel

Blood level of thyroid

hormone falls below

a set point.

Thyroid hormone

is secreted.

yroid Gland

Anterior Pituitary

Hypothalamus

Stimulus Response

+

Rise in the blood

level of thyroid

hormone inhibits

secretion of TRH

and TSH.

TRH

TSH

Figure 15.6 a negative feedback loop controls the secretion of thyroid hormone. (© Cengage Learning)

© Cengage Learning/Gary Head

15.6

294 Chapter 15

parathyroid glands Four

small endocrine glands

behind the thyroid gland.

Parathyroid hormone (PTH)

regulates blood calcium

levels.

thyroid gland Endocrine

gland that produces thy-

roid hormone (TH), which

is required for normal

metabolism, growth, and

development.

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