Human Physiology, 14th edition (2016)

Endocrine Glands 341

The thyroid gland is located just below the larynx ( fig. 11.21 ).

Its two lobes are positioned on either side of the trachea and are

connected anteriorly by a medial mass of thyroid tissue called

the isthmus. The thyroid is the largest of the purely endocrine

glands, weighing 20 to 25 grams.

On a microscopic level, the thyroid gland consists of

numerous spherical hollow sacs called thyroid follicles

( fig.  11.22 ). These follicles are lined with a simple cuboidal

epithelium composed of follicular cells that synthesize the prin-

cipal thyroid hormone, thyroxine. The interior of the follicles

contains colloid, a protein-rich fluid. In addition to the follicu-

lar cells that secrete thyroxine, the thyroid also contains para-

follicular cells that secrete a hormone known as calcitonin (or

thyrocalcitonin ).

the actions of anabolic hormones, including growth hormone and
insulin. Chronic stress, with its prolonged high secretion of gluco-
corticoids, can thereby aggravate insulin resistance —the reduced
sensitivity of target tissues to insulin. Stress can thus make treat-
ment of diabetes difficult, and can contribute to a constellation of
symptoms associated with type 2 diabetes mellitus. Diabetes, and
the “metabolic syndrome” associated with it, are discussed more
fully in chapter 19, section 19.4.

Clinical Investigation CLUES

Rachel’s physician said she previously had somewhat

high blood glucose, but now her blood glucose tested low.

• How might her blood tests be related to her taking
  prednisolone?


• How might stopping prednisolone abruptly be
  related to her blood test results?

| CHECKPOINT

8a. List the categories of corticosteroids and identify

the zone of the adrenal cortex that secretes the

hormones within each category.

8b. Identify the hormones of the adrenal medulla and

describe their effects.

9a. Explain how the secretions of the adrenal cortex and

adrenal medulla are regulated.

9b. Explain how stress affects the secretions of

the adrenal cortex and medulla. Why does

hypersecretion of the adrenal medullary hormones

make a person more susceptible to disease?

11.5 Thyroid and Parathyroid Glands

The thyroid secretes thyroxine (T 4 ) and triiodothyronine (T 3 ),

which are needed for proper growth and development and

which are primarily responsible for determining the basal met-

abolic rate (BMR). The parathyroid glands secrete parathyroid

hormone, which helps to raise the blood Ca^2 1 concentration.

LEARNING OUTCOMES

After studying this section, you should be able to:

10. Describe the structure of the thyroid gland, the
    production and actions of the thyroid hormones, and
    disorders of thyroid function.


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

Thyroid

gland

Trachea

Cricoid

cartilage

of larynx

Thyroid

cartilage

of larynx

Figure 11.21 The thyroid gland. Its relationship to the

larynx and trachea.

Figure 11.22 A photomicrograph (250 3  ) of a

thyroid gland. Numerous thyroid follicles are visible. Each

follicle consists of follicular cells surrounding the fluid known as

colloid, which contains thyroglobulin.

Colloid Follicles

Follicular

cells