Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 19 Adrenocortical Function
occur simultaneously ( Nothelfer and Weinhold, 1992 ; Van
Sluijs et al ., 1995 ). However useful this measurement may
be, in practice it is not the first choice when it comes to the
differential diagnosis of hyperadrenocorticism because of
the requirements for collecting and handling the samples
and the technically difficult assay (see also Section IV.E).
In contrast to the determination of endogenous ACTH,
the HDDST is readily available. Despite a decreased sensitivity
to the suppression by glucocorticoids, the ACTH
secretion of most animals with pituitary-dependent hyperadrenocorticism
can be suppressed with a 10-fold dose
of 0.1 mg dexamethasone/kg body weight, resulting in a
decrease of the adrenocortical secretion. The autonomous
hypersecretion by adrenocortical tumors will not be influenced
by the high dose of dexamethasone.
Two procedures are used in the dog, one employing
plasma cortisol as a reflection of adrenocortical secretion
(Meijer et al ., 1979 ) and the other urinary corticoid/creatinine
ratios ( Rijnberk et al ., 1988 ). In both, a greater than
50% decline from baseline values is regarded as diagnostic
for pituitary-dependent hyperadrenocorticism (see Section
IV.F). When suppression is less than 50%, the hyperadrenocorticism
may be due to either an adrenocortical tumor
or a pituitary ACTH excess that is extremely resistant to
dexamethasone suppression. For the differentiation between
these two forms, the above-mentioned measurement of
endogenous ACTH may be necessary, or a CRH test may be
performed. In dogs with pituitary-dependent hyperadrenocorticism,
CRH administration (1 μ g/kg I.V.) elevates both
plasma ACTH and cortisol, whereas no such rise is seen in
dogs with hyperadrenocorticism caused by adrenocortical
tumors ( Van Wijk et al ., 1994 ). The high cost of the commercially
available powder preparation and the necessary
pharmaceutical preparation prohibit general use of CRH.
E . Collection and Handling of Samples
It has become common practice for clinicians to perform
endocrine function studies and to mail samples to a laboratory
for measurement of cortisol. It is therefore important to
rule out nonpathological factors that can alter hormone concentrations.
Apart from stress (see Section IV.B), the nutritional
state of the animal may also play a role. In a study by
Reimers et al . (1986) , the researchers found that mean serum
concentrations of cortisol in dogs fasted 12 or 24 h were
lower than those in dogs that were not fasted. The concentrations
were not further affected by continued fasting for 36 h.
Olson et al . (1981) have studied several aspects of sample
handling, such as uncentrifuged storage and storage
time. They concluded that either serum or plasma of dogs
is suitable for radioimmunoassay of cortisol, and samples
(with and without added anticoagulants) may be left uncentrifuged
at 4 ° C for up to 40 h without cortisol degradation.
However, prolonged storage of serum at room temperature
is detrimental, particularly for samples that have large concentrations
of cortisol. Samples allowed to defrost and sit for
more than 3 days en route to a laboratory may have a lower
cortisol concentration than they did at the time of collection.
The peptide ACTH is readily inactivated at room
temperature. Therefore, blood for ACTH measurements
should be collected in EDTA-coated tubes placed in ice,
and the blood should be centrifuged at 4 ° C. The plasma
should then be placed in polypropylene tubes and kept frozen
until assayed. These requirements are most easily fulfilled
when the samples are collected in a clinic with the
necessary facilities. If the samples have to be transformed,
it is imperative that they be kept frozen with dry ice in a
Styrofoam container. These strict rules can be alleviated to
some extent by adding a protease inhibitor (aprotinin) to
the collection tubes ( Kemppainen et al ., 1994 ).
F . Protocols
The protocols presented next are applicable to the dog.
With the data presented in some of the tables, extrapolation
to other species may be possible.
1 . ACTH Stimulation Test
a . Indications
The ACTH stimulation test is performed when there is suspicion
of decreased adrenocortical reserve capacity: (1)
primary adrenocortical insufficiency (Addison’s disease)
and (2) (iatrogenic) secondary adrenocortical insufficiency.
b . Performance
Blood for cortisol measurements is collected immediately
before and 90 min after intravenous administration of 0.25 mg
synthetic ACTH (Cortrosyn [Organon]). In cases in which
treatment for adrenocortical insufficiency was already started,
on the morning of the test, the cortisone administration is
postponed until after completion of the test. When the treatment
is already longer than 3 to 4 days, iatrogenic suppression
of the adrenals should be taken into consideration.
c . Interpretation
In healthy dogs, the cortisol concentrations rise to 270 to
690nmol/liter ( Rijnberk, 1996 ). In Addison’s disease, the
control value is usually low and does not increase following
ACTH administration. In animals with secondary adrenocortical
insufficiency, the basal cortisol values may be low as
well, and, depending on the severity (duration) of the ACTH
deficiency, the cortisol rise is subnormal or absent.
2 . Low-Dose Dexamethasone Suppression Test
a . Indication
The low-dose dexamethasone suppression test is used when
hyperadrenocorticism (Cushing’s syndrome) is suspected.