| Summary: | Abstract Background Occult (or atypical) hyperadrenocorticism (HAC) shows clinical signs and laboratory abnormalities similar to classic hyperadrenocorticism, but normal signs in routine screening tests such as the corticotropin (ACTH) stimulation test and low‐dose dexamethasone suppression test (LDDST). Here, we describe a case of occult HAC in a Yorkshire terrier treated with mitotane. Case An 11‐year‐old spayed female presented to the Veterinary Teaching Hospital of Seoul National University because of respiratory distress symptoms, polyphagia, and polydipsia, suggestive of HAC. In abdominal sonography, enlargement of the caudal pole of the left adrenal gland was found, but the cortisol level of post‐ACTH stimulation test was below the cut‐off value, and LDDST was negative. To finalise the diagnosis of occult HAC, 17‐hydroxyprogesterone (17‐OHP) was examined. The concentrations of 17‐OHP (pre‐ and post‐ACTH stimulation) were found to be elevated. As occult HAC was highly suspected, we prescribed trilostane for trial therapy. At first, the clinical signs improved, but they later worsened. We changed medication as trilostane to mitotane, and the symptoms were relieved after mitotane administration. Conclusion This is a unique case of occult HAC in which the response to mitotane was better than trilostane.
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