A comparison of
cortisol concentrations, determined by high-performance liquid chromatography (HPLC) and radioimmunoassay, was made in healthy, mixed-breed dogs and in dogs with clinical signs of
Cushing's syndrome. Absolute concentration of
cortisol was less in resting,
dexamethasone-suppressed, and
ACTH-stimulated plasma samples, when measured by HPLC relative to
cortisol concentrations determined by radioimmunoassay. Diagnosis of
Cushing's syndrome could be made using either method. Plasma concentrations of
cortisone and
corticosterone were determined by HPLC.
Cortisone and
corticosterone concentrations alone were not diagnostic of
Cushing's syndrome; but when used in conjunction with
cortisol determinations, they provided additional criteria for laboratory confirmation of the clinical diagnosis of
Cushing's syndrome. The clearance of exogenous
dexamethasone used in the
dexamethasone-suppression test was examined in the normal and cushingoid dogs. Plasma concentrations of exogenous
dexamethasone were constant during the 3-hour test in the normal dogs. However, in 75% of the dogs with
Cushing's syndrome, exogenous
dexamethasone was cleared completely in the 3-hour test. Thus,
dexamethasone clearance rates were excessively fast in most dogs with
Cushing's syndrome.