Endogenous ACTH concentration
In dogs with confirmed hypoadrenocorticism that have normal electrolyte concentrations, measurement of an endogenous (basal)
ACTH concentration is useful in identifying whether hypoadrenocorticism is primary or secondary. Measurement of an ACTH concentration
above the reference range confirms a diagnosis of primary hypoadrenocorticism, while an ACTH concentration within or below
the reference range is consistent with a diagnosis of secondary hypoadrenocorticism.
In dogs without the classic electrolyte changes of hypoadrenocorticism, differentiation of primary from secondary hypoadrenocorticism
could potentially allow identification of dogs that are at risk for progression to complete adrenal failure and, therefore,
require mineralocorticoid as well as glucocorticoid supplementation.
Interestingly, in one retrospective study of 11 dogs with glucocorticoid-deficient hypoadrenocorticism, only one ultimately
developed mineralocorticoid deficiency, despite the fact that most of the dogs (9/11) were diagnosed as having primary hypoadrenocorticism.
Diagnosis of hypoadrenocorticism historically has relied on the cortisol concentration as an indicator of adrenal reserve,
while plasma aldosterone concentrations have largely been ignored, primarily because plasma aldosterone assays have not been
widely available. However, cortisol concentrations may not necessarily parallel aldosterone concentrations, particularly in
dogs with glucocorticoid-deficient hypoadrenocorticism.
Aldosterone might theoretically be useful in distinguishing primary from secondary hypoadrenocorticism and pure glucocorticoid
deficiency from classic primary hypoadrenocorticism. In one study, however, plasma aldosterone concentrations were not useful
in distinguishing these three groups of dogs.
Aldosterone-to-renin and cortisol-to-adrenocorticotrophic hormone ratios
Measurement of cortisol-to-ACTH ratio (CAR) and aldosterone-to-renin ratio (ARR) has been proposed as an alternative diagnostic
test for hypoadrenocorticism in dogs.
In 22 dogs with primary hypoadrenocorticism and 60 normal dogs, there was overlap between the groups for aldosterone, renin,
cortisol and ACTH concentrations; however, the CAR and ARR was consistently lower in dogs with hypoadrenocorticism than in
This suggests that these ratios potentially could be used in place of the ACTH stimulation test for diagnosis of hypoadrenocorticism.
Advantages of measuring these ratios include the need for collection of only one blood sample and avoiding the cost of performing
the ACTH stimulation test.
Disadvantages include the lack of availability of the renin and aldosterone assay and the issues of sample handling with regard
to measurement of endogenous ACTH.
More studies are needed to evaluate the CAR and ARR in dogs with other illnesses that might undergo testing for hypoadrenocorticism.
The CAR ratio in secondary hypoadrenocorticism also might overlap with normal dogs.
Dr. Hoskins is owner of Docu-Tech Services. He is a diplomate of the American College of Veterinary Internal Medicine with
specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200 or e-mail: