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