The Endocrine System
Mechanisms of Hormone Action
I. The great majority of receptors for steroid and thyroid
hormones are inside the target cells; those for the peptide
hormones and catecholamines are on the plasma membrane.
II. Hormones can cause up-regulation and down-regulation
of their own receptors and those of other hormones. The
induction of one hormone’s receptors by another hormone
increases the ﬁ rst hormone’s effectiveness and may be essential
to permit the ﬁ rst hormone to exert its effects.
III. Receptors activated by peptide hormones and catecholamines
utilize one or more of the signal transduction pathways
available to plasma-membrane receptors; the result is altered
membrane potential or protein activity in the cell.
IV. Intracellular receptors activated by steroid and thyroid hormones
function as transcription factors, combining with DNA in the
nucleus and inducing the transcription of DNA into mRNA; the
result is increased synthesis of particular proteins.
V. In pharmacological doses, hormones can have effects not seen
under ordinary circumstances.
Inputs that Control Hormone Secretion
I. The secretion of a hormone may be controlled by the plasma
concentration of an ion or nutrient that the hormone regulates,
by neural input to the endocrine cells, and by one or more
II. The autonomic nervous system is the neural input controlling
many hormones. Neuron endings from the sympathetic and
parasympathetic nervous systems terminate directly on cells
within some endocrine glands, thereby regulating hormone
Types of Endocrine Disorders
I. Endocrine disorders may be classiﬁ ed as hyposecretion,
hypersecretion, and target-cell hypo- or hyperresponsiveness.
a. Primary disorders are those in which the defect is in the
cells that secrete the hormone.
b. Secondary disorders are those in which there is too much or
too little tropic hormone.
c. Hyporesponsiveness is due to an alteration in the receptors
for the hormone, to disordered postreceptor events, or to
failure of normal metabolic activation of the hormone in
cases requiring such activation.
II. These disorders can be distinguished by measurements of the
hormone and any tropic hormones under both basal conditions
and during experimental stimulation of the hormone’s secretion.
SECTION A KEY TERMS
SECTION A CLINICAL TERMS
SECTION A REVIEW QUESTIONS
1. What are the three general chemical classes of hormones?
2. Which catecholamine is secreted in the largest amount by the
adrenal medulla, and why?
3. What are the major hormones produced by the adrenal cortex?
By the testes? By the ovaries?
4. Which classes of hormones are carried in the blood mainly
as unbound, dissolved hormone? Mainly bound to plasma
5. Do protein-bound hormones diffuse out of capillaries?
6. Which organs are the major sites of hormone excretion and
7. How do the rates of metabolism and excretion differ for the
various classes of hormones?
8. List some metabolic transformations that prohormones and
some hormones must undergo before they become biologically
9. Contrast the locations of receptors for the various classes of
10. How do hormones inﬂ uence the concentrations of their own
receptors and those of other hormones? How does this explain
permissiveness in hormone action?
11. Describe the sequence of events when peptide or catecholamine
hormones bind to their receptors.
12. Describe the sequence of events when steroid or thyroid
hormones bind to their receptors.
13. What are the direct inputs to endocrine glands controlling
14. How does control of hormone secretion by plasma mineral
ions and nutrients achieve negative feedback control of these
15. What roles does the autonomic nervous system play in
controlling hormone secretion?
16. What groups of hormone-secreting cells receive input from
neurons located in the brain rather than in the autonomic
17. How would you distinguish between primary and secondary
hyposecretion of a hormone? Between hyposecretion and