Endocrine Glands 317Interactions between the various hormones produce effects
that may be synergistic, permissive, or antagonistic.
Rachel has been taking prednisolone for some time as a
treatment for her arthritis, but abruptly stopped because it
was making her look “puffy.” Three weeks later she went to
a physician because she had difficulty sleeping, was always
uncomfortably warm, and generally felt poorly. The physi-
cian noticed that she was sweaty and had a rapid pulse and
high blood pressure. He also noticed that her eyes seemed
to bulge. He said she might have a pheochromocytoma, but
needed to test more-likely possibilities. He was surprised
that her blood glucose was low, because previously it had
tested somewhat high. He said the other results suggested
that her symptoms were due to Graves’ disease.
Some of the new terms and concepts you will
encounter include:- TSH, thyroxine, and Graves’ disease
- ACTH, corticosteroids, glucocorticoids, and
Addison’s disease - Adrenal medulla, catecholamines, and
pheochromocytoma
Clinical Investigation
LEARNING OUTCOMESAfter studying this section, you should be able to:- Describe the chemical nature of hormones and
define the terms prehormone and prohormone. - Describe the different types of hormone interactions
and the significance of hormone concentrations.
Figure 11.1 The major endocrine glands.
( a ) The anatomical location of some of the endocrine
glands. ( b ) A photomicrograph of a pancreatic islet
(of Langerhans) within the pancreas.(a)Pancreatic islet
(of Langerhans)(b)Thyroid glandAdrenal glandPancreasOvaryTestisPituitary
glandPineal glandHypothalamusEndocrine glands lack the ducts that are present in exocrine
glands (chapter 1, section 1.3). The endocrine glands secrete
their products, which are biologically active molecules called
hormones, into the blood. The blood carries the hormones to tar-
get cells that contain specific receptor proteins for the hormones,
and which therefore can respond in a specific fashion to them.
Many endocrine glands are organs whose primary functions are
the production and secretion of hormones ( fig. 11.1 a ). The pan-
creas functions as both an exocrine and an endocrine gland; the
endocrine portion of the pancreas is composed of clusters of cells
called the pancreatic islets (islets of Langerhans) ( fig. 11.1 b ). The
concept of the endocrine system, however, must be extended
beyond these organs because many other organs in the body
secrete hormones. These organs may be categorized as endocrine
glands even though they serve other functions as well. It is appro-
priate, then, that a partial list of the endocrine glands ( table 11.1 )
should include the heart, liver, adipose tissue, and kidneys.
Some specialized neurons, particularly in the hypothalamus,
secrete chemical messengers into the blood rather than into a11.1 Endocrine Glands and Hormones
Hormones are regulatory molecules secreted into the blood
by endocrine glands. Chemical categories of hormones
include steroids, amines, polypeptides, and glycoproteins.