A two and a half year old border collie cross was referred for several episodes of collapse, weakness and noticeable blanching of her oral mucous membranes. Quick assessment tests demonstrated mild hypoglycemia. Resting blood pressure measurements showed intermittent hypotension and thoracic radiographs revealed the persistence of a previously diagnosed megaesophagus. Diagnostic testing showed glucocorticoid deficiency and complete resolution of collapsing episodes following supplementation. Unique features of this case included the dog?s previous history of immune-mediated thyroiditis and a negative acetylcholine receptor antibody titer.