Guyton and Hall
CHAPTER 75 – Introduction to Endocrinology
Many of the body’s chemical messenger systems interact with one another to maintain
homeostasis.
Endocrine hormones are carried by the circulatory system to cells throughout the body,
including the nervous system. Some endocrine hormones affect many different types of cells
of the body and other hormones affect mainly target tissues because these tissues have
abundant receptors for the hormone.
The multiple hormone systems play a key role in regulating almost all body functions
metabolism, growth and development, water and electrolyte balance, reproduction, and
behavior.
Three general classes of hormones exist:
Proteins and polypeptides:
Stored in secretory vesicles until needed a stimulus for hormone secretion often
involves changes in the intracellular calcium or changes in cyclic adenosine
monophosphate (cAMP) in the cell; exocytosis
Steroid hormones:
Hormones that are lipid soluble (chemical structure is similar to that of cholesterol)
usually little hormone storage; large stores of cholesterol esters in cytoplasm
vacuoles can be rapidly mobilized for steroid synthesis after a stimulus.
Amine hormones (derived from amino acid tyrosine):
o Thyroid hormones:
Thyroid hormones are synthesized and stored in thyroid gland and
incorporated into macromolecules of the protein thyroglobulin (stored in
large follicles in gland) amines split from thyroglobulin hormone
secretion (free hormones in the bloodstream) combination with plasma
proteins (thyroxine-binding globulin) slow release of the hormones to
target tissues.
o Adrenal medullary hormones:
Epinephrine and norepinephrine stored as catecholamines in secretory
granules (just like protein hormones)
Each hormone has its own characteristic onset and duration of action tailored to perform
its specific control function.
Negative feedback mechanisms ensure a proper level of hormone activity at the target
tissue after a stimulus causes release of a hormone, conditions or products resulting from
the action of the hormone tend to suppress its further release: the hormone (or one of its
products) has a negative feedback effect to prevent oversecretion or overactivity.
Only when the target tissue activity rises to an appropriate level will feedback signals
to the endocrine gland become powerful enough to slow further secretion of the
hormone.
CHAPTER 75 – Introduction to Endocrinology
Many of the body’s chemical messenger systems interact with one another to maintain
homeostasis.
Endocrine hormones are carried by the circulatory system to cells throughout the body,
including the nervous system. Some endocrine hormones affect many different types of cells
of the body and other hormones affect mainly target tissues because these tissues have
abundant receptors for the hormone.
The multiple hormone systems play a key role in regulating almost all body functions
metabolism, growth and development, water and electrolyte balance, reproduction, and
behavior.
Three general classes of hormones exist:
Proteins and polypeptides:
Stored in secretory vesicles until needed a stimulus for hormone secretion often
involves changes in the intracellular calcium or changes in cyclic adenosine
monophosphate (cAMP) in the cell; exocytosis
Steroid hormones:
Hormones that are lipid soluble (chemical structure is similar to that of cholesterol)
usually little hormone storage; large stores of cholesterol esters in cytoplasm
vacuoles can be rapidly mobilized for steroid synthesis after a stimulus.
Amine hormones (derived from amino acid tyrosine):
o Thyroid hormones:
Thyroid hormones are synthesized and stored in thyroid gland and
incorporated into macromolecules of the protein thyroglobulin (stored in
large follicles in gland) amines split from thyroglobulin hormone
secretion (free hormones in the bloodstream) combination with plasma
proteins (thyroxine-binding globulin) slow release of the hormones to
target tissues.
o Adrenal medullary hormones:
Epinephrine and norepinephrine stored as catecholamines in secretory
granules (just like protein hormones)
Each hormone has its own characteristic onset and duration of action tailored to perform
its specific control function.
Negative feedback mechanisms ensure a proper level of hormone activity at the target
tissue after a stimulus causes release of a hormone, conditions or products resulting from
the action of the hormone tend to suppress its further release: the hormone (or one of its
products) has a negative feedback effect to prevent oversecretion or overactivity.
Only when the target tissue activity rises to an appropriate level will feedback signals
to the endocrine gland become powerful enough to slow further secretion of the
hormone.
