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U N I T 9
Endocrine System
Real progress in measuring plasma hormone levels
came more than 40 years ago with the use of competi-
tive binding and the development of radioimmunoassay
(RIA) methods. This method uses a radiolabeled form
of the hormone and a hormone antibody that has been
prepared by injecting an appropriate animal with a puri-
fied form of the hormone. The unlabeled hormone in
the sample being tested competes with the radiolabeled
hormone for attachment to the binding sites of the anti-
body. Measurement of the radiolabeled hormone–anti-
body complex then provides a means of arriving at a
measure of the hormone level in the sample. Because
hormone binding is competitive, the amount of radio-
labeled hormone–antibody complex that is formed
decreases as the amount of unlabeled hormone in the
sample is increased. Newer techniques of RIA have been
introduced, including the immunoradiometric assay
(IRMA). IRMA uses two antibodies instead of one.
These two antibodies are directed against two different
parts of the molecule, and therefore IRMA assays are
more specific. RIA has several disadvantages, including
limited shelf life of the radiolabeled hormone and the
cost for the disposal of radioactive waste.
Nonradiolabeled methods have been developed in
which the antigen of the hormone being measured is
linked to an enzyme-activated label (e.g., fluorescent
label, chemiluminescent label) or latex particles that
can be agglutinated with an antigen and measured. The
enzyme-linked immunosorbent assays (ELISAs) use
antibody-coated plates and an enzyme-labeled reporter
antibody. Binding of the hormone to the enzyme-labeled
reporter antibody produces a colored reaction that can
be measured using a spectrophotometer.
In many situations, immunoassays are unreliable or
unavailable. For some steroid or peptide hormones,
mass spectrometry is becoming increasingly useful and
can be combined with other analytical techniques, such
as liquid chromatography. These approaches provide
definitive identification of the relevant hormone or
compound according to its chemical or physical char-
acteristics (e.g., unequivocal detection of performance-
enhancing agents in sports).
Other blood tests that are routinely used in endo-
crine disorders include various autoantibodies. For
example, antithyroid peroxidase (anti-TPO) antibod-
ies are measured during the initial diagnostic workup
and subsequent follow-up of patients with Hashimoto
thyroiditis. Other endocrine disorders that use autoan-
tibody testing include type 1 diabetes, Graves disease,
autoimmune hypoparathyroidism, and autoimmune
Addison disease.
UrineTests
Measurements of urinary hormone or hormone metab-
olites often are done on a 24-hour urine sample and
provide a better measure of hormone levels during that
period than hormones measured in an isolated blood
sample. The advantages of a urine test include the rela-
tive ease of obtaining urine samples and the fact that
blood sampling is not required. The disadvantage is
that reliably timed urine collections often are difficult
to obtain. For example, a person may be unable to uri-
nate at specific timed intervals, and urine samples may
be accidentally discarded or inaccurately preserved.
Because many urine tests involve the measurement of
a hormone metabolite rather than the hormone itself,
drugs or disease states that alter hormone metabolism
may interfere with the test result. Some urinary hormone
metabolite measurements include hormones from more
than one source and are of little value in measuring hor-
mone secretion from a specific source. For example, uri-
nary 17-ketosteroids are a measure of both adrenal and
gonadal androgens.
Stimulation and SuppressionTests
Stimulation tests are used when hypofunction of an
endocrine organ is suspected. Atropic or stimulating
hormone can be administered to test the capacity of an
endocrine organ to increase hormone production. The
capacity of the target gland to respond is measured by an
increase in the appropriate hormone. For example, the
function of the hypothalamic-pituitary-adrenal system
can be evaluated through stimulation tests using ACTH
and measuring the cortisol response. Failure to increase
cortisol levels after a ACTH stimulation test suggests an
inadequate capacity to produce cortisol by the adrenals
(i.e., the adrenal is dysfunctional in some way).
Suppression tests are used when hyperfunction of an
endocrine organ is suspected. When an organ or tissue
is functioning autonomously (i.e., is not responding to
the normal negative feedback control mechanisms and
continues to secrete excessive amounts of hormone), a
suppression test may be useful to confirm the situation.
For example, when a GH-secreting tumor is suspected,
the GH response to a glucose load is measured as part
of the diagnostic workup (see Chapter 32). Normally,
a glucose load would suppress GH levels. However, in
adults with GH-secreting tumors (a condition known as
acromegaly
), GH levels are not suppressed (and para-
doxically increase in 50% of cases).
GeneticTests
The diagnosis of genetic diseases using DNA analysis
is rapidly becoming a routine part of endocrine prac-
tice. Completion of the human genome sequence has
revealed the presence of about 30,000 genes. The con-
siderable interest in the field of genomics (i.e., examina-
tion of the DNA) and transcriptomics (i.e., examination
of the mRNA) has been complemented by advances in
proteomics (i.e., examination of the proteome, which is
all of the proteins expressed by a cell or tissue type). It
is proposed that compared with the size of the genome,
the proteome is far larger, with several hundred thou-
sand to several million different protein forms possible.
Analysis of the proteins produced by normal and abnor-
mal endocrine cells, tissues, and organs will lead to a
better understanding of the pathophysiologic processes
of endocrine conditions. This may also lead to selective
targeting for new drug development.
