A 3-year-old intact female guinea pig presented with bilateral, non-inflammatory alopecia on the abdomen and flanks, with no other complaints. Ultrasound diagnostics showed signs of bilateral polycystic ovaries and endometrial hyperplasia (neoplasia could not be ruled out). An ovariohysterectomy was performed, and a uterine sample sent for histopathology confirmed glandular-cystic endometrial hyperplasia. Six months later, the patient returned to the clinic with complaints of recurrent non-inflammatory alopecia on the left side, decreased appetite, and soft stool. Diagnostics included a CBC, biochemistry, and abdominal ultrasound. Blood tests showed no abnormalities. Ultrasound revealed left-sided adrenal hyperplasia. An ACTH stimulation test confirmed hyperadrenocorticism. The guinea pig was started on Trilostane at 2 mg/kg every 12 hours, with no side effects reported. One month later, a follow-up examination showed full skin recovery. Ultrasound revealed no changes in the adrenal gland size or visualization, and cortisol levels remained elevated on the ACTH stimulation test. One month later, cortisol levels on the ACTH stimulation test fell within the conditionally reference range, leading to a decision to begin reducing the Trilostane dose. Two years later, the guinea pig presented with decreased appetite, reduced stool production, and lethargy. Diagnostics included CBC, biochemistry, abdominal X-rays, and ultrasound. Ultrasound detected a right adrenal tumor measuring. The guinea pig was stabilized in a clinical setting. A decision was made to perform a right adrenalectomy. Within 24 hours post-operation, cardiac arrest and death were diagnosed. Necropsy revealed a thrombus in the caudal vena cava. Histopathology of the adrenal tumor confirmed a carcinoma.