Human Physiology, 14th edition (2016)

344 Chapter 11

CLINICAL APPLICATION

Graves’ disease is an autoimmune disorder (caused by the

person’s own immune system; chapter 15, section 15.6) in

which autoantibodies bind to the receptors for TSH on thy-

roid gland cells. This causes growth of the thyroid (a goiter)

and stimulates the excessive secretion of thyroid hormones.

Although TSH levels are very low because of negative feed-

back, the thyroid continues to be stimulated by the thyrotropin

receptor antibodies to grow and secrete thyroxine. Unlike peo-

ple with endemic goiter caused by iodine deficiency, who are

hypothyroid, those with a goiter caused by Graves’ disease are

hyperthyroid. The hyperthyroidism produces such symptoms

as sensitivity to heat, palpitations, and others (see table 11.8 ).

Many people with Graves’ disease also have Graves’ opthal-

mopathy, where anatomical changes in the orbit of the eyes

produce bulging of the eyes, or exophthalmos ( fig. 11.26 ).

Graves’ disease is the most common cause of hyperthyroid-

ism and is 5 to 10 times more common in women than men.

Feature Hypothyroid Hyperthyroid

Growth and

development

Impaired growth Accelerated growth

Activity and sleep Lethargy; increased

sleep

Increased activity;

decreased sleep

Temperature

tolerance

Intolerance to cold Intolerance to heat

Skin characteristics Coarse, dry skin Normal skin

Perspiration Absent Excessive

Pulse Slow Rapid

Gastrointestinal

symptoms

Constipation;

decreased

appetite; increased

weight

Frequent bowel

movements;

increased appetite;

decreased weight

Reflexes Slow Rapid

Psychological

aspects

Depression and

apathy

Nervous,“emotional”

state

Plasma T 4 levels Decreased Increased

Table 11.8 | Comparison of
Hypothyroidism and Hyperthyroidism

Figure 11.26 A woman with Graves’ disease. She

exhibits exophalmos (protruding eyes) and has a goiter due

to overstimulation of the thyroid gland by autoantibodies that

activate the TSH receptors.

Parathyroid Glands

The small, flattened parathyroid glands are embedded

in the posterior surfaces of the lateral lobes of the thyroid

gland, as shown in figure 11.27. There are usually four para-

thyroid glands: a superior and an inferior pair, although the

precise number can vary. Each parathyroid gland is a small

yellowish-brown body 3 to 8 mm (0.1 to 0.3 in.) long, 2 to

5 mm (0.07 to 0.2 in.) wide, and about 1.5 mm (0.05 in.)

deep.

Parathyroid hormone (PTH) is the only hormone

secreted by the parathyroid glands. PTH, however, is the sin-

gle most important hormone controlling the calcium concen-

trations in the blood. It promotes a rise in blood calcium levels

by acting on the bones, kidneys, and intestine ( fig.  11.28 ).

Regulation of calcium balance is described in chapter 19,

section 19.6.

Clinical Investigation CLUES

Rachel’s eyes bulged and she had symptoms that the

physician said were a result of Graves’ disease.

• What must be true of her TSH and thyroxine levels
  to justify this diagnosis?


• How does the diagnosis relate to her bulging eyes
  and other symptoms?

| CHECKPOINT

10a. Describe the structure of the thyroid gland and list

the effects of thyroid hormones.

10b. Describe how thyroid hormones are produced and

how their secretion is regulated.

10c. Explain the consequences of an inadequate dietary

intake of iodine.

11. Describe the location of the parathyroid glands and
    the actions of parathyroid hormone.