C h a p t e r 3 1
Mechanisms of Endocrine Control
765
The cloning of many endocrine system genes has had
an enormous impact on everyday clinical practice. For
example, identification of a gene for a given disorder
(e.g., the
RET
proto-oncogene in certain multiple endo-
crine neoplasia syndromes) means that faster diagno-
sis and more appropriate management for the affected
individual can occur, but also that screening of family
members for kindred harboring a known mutation can
be undertaken.
ImagingTechniques
Imaging studies are important in the diagnosis and
follow-up of endocrine disorders. Imaging modalities
related to endocrinology can be divided into isotopic and
nonisotopic types. Isotopic imaging includes radioactive
scanning of the thyroid (e.g., using radioiodine) and
parathyroids (e.g., using sestamibi). Nonisotopic imag-
ing includes magnetic resonance imaging (MRI), which
is the preferred choice for pituitary and hypothalamic
imaging, and computed tomography (CT) scanning,
which is preferred for adrenal lesions and abdomi-
nal endocrine lesions. Ultrasonographic scanning pro-
vides excellent and reproducible anatomic images for
the thyroid, parathyroids, and neighboring structures.
Thyroid ultrasonography is recommended for managing
thyroid nodules and can aid in visualization of the nod-
ule for biopsy (fine needle aspiration), which is neces-
sary to help distinguish benign from malignant etiology.
Selective venography is usually accompanied by venous
sampling to determine hormonal output from a gland
or organ (e.g., adrenal, pituitary, and kidney). Positron
emission tomography (PET) scanning is being used
more widely for evaluation of endocrine tumors. Dual
electron x-ray absorptiometry (DEXA) is used routinely
for the diagnosis and monitoring of osteoporosis and
metabolic bone diseases.
R E V I EW E X E R C I S E S
1.
Vitamin D is often considered a hormone rather
than a vitamin.
A.
Explain.
2.
Thyroid hormones are transported in the serum
bound to transport proteins such as thyroid-
binding globulin and albumin.
A.
Explain why free thyroxine (T
4
) levels are
usually used to assess thyroid function rather
than total T
4
levels.
3.
People who are being treated with exogenous
forms of corticosteroid hormones often experience
diminished levels of ACTH and exogenously
produced cortisol.
A.
Explain using information regarding the
hypothalamic-pituitary feedback control of
cortisol production by the adrenal cortex.
B I B L I O G R A P H Y
Alberts B, Aranda A, Pascual A. Nuclear hormone receptors and
gene expression.
Physiol Rev.
2001;81(3):1239–1304.
Alberts B, Johnson A, Lewis V, et al.
Molecular Biology of the Cell
.
5th ed. New York, NY: Garland Science; 2008:889–903.
SUMMARY CONCEPTS
■■
The endocrine system acts as a communication
system that uses hormones as chemical
messengers. Hormones travel through the blood
to distant target sites of action, or act locally as
paracrine or autocrine messengers to incite more
local effects.
■■
Hormones exert their effects by interacting with
high-affinity receptors, which in turn are linked
to one or more effector systems in the cell. Some
hormone receptors are located on the surface
of the cell and act through second messenger
mechanisms, and others are located in the cell,
where they modulate the synthesis of enzymes,
transport proteins, or structural proteins. Other
hormones bind to nuclear receptors and act by
directly regulating gene transcription.
■■
Many of the endrocrine glands are under the
regulatory control of other parts of the endocrine
and nervous systems.The hypothalamus and
the pituitary gland form a complex integrative
network that joins the nervous system and the
endocrine system; this central network controls
the output from many of the other glands in the
body.
■■
Many hormones are controlled by negative
feedback loops that adjust their level and confer
stability to the many functions of the endocrine
system.
■■
Endocrine function can be assessed directly by
measuring blood hormone levels or indirectly
by assessing the effects that a hormone has on
the body (e.g., assessment of insulin function
through blood glucose). Imaging techniques
are increasingly used to visualize endocrine
structures, and genetic techniques are used
to determine the presence of genes that
contribute to the development of endocrine
disorders.
