A 28‑year‑old man with Hashimoto's thyroiditis had repeated severe low‑blood‑sugar episodes caused by a sudden loss of ACTH, the hormone that tells the adrenal glands to make cortisol. Tests showed his pituitary was otherwise normal, and imaging showed small adrenal glands. He was treated with hydrocortisone and stopped having hypoglycemia. The case suggests that isolated ACTH deficiency, though rare, can cause dangerous low blood sugar and should be considered in similar situations.

Our patient is a 28-year-old male who was being treated by a local doctor for Hashimoto's thyroiditis. Four days prior to admission, nausea and diarrhea appeared, and it gradually became difficult for him to eat. The night before admission, his level of consciousness decreased (Japan Coma Scale (JCS) II-20), and he was rushed to the hospital. His other vital signs were stable. After completing a detailed examination, the cause was diagnosed as hypoglycemia with a blood glucose level of 21 mg/dl. After the administration of glucose, he regained consciousness and became alert, allowing him to return home, with the expectation that he would return to the hospital for a follow-up visit at a later date. However, an altered consciousness (JCS I-3) appeared again the next morning. Similar to the previous day, the cause of the altered consciousness was determined to be hypoglycemia. After making a detailed inpatient examination, his early morning fasting serum cortisol level was found to be less than 0.1 &#x3bc;g/dL, and his blood adrenocorticotropic hormone (ACTH) was 3.1 pg/mL, thus indicating decreases in both. After performing rapid ACTH testing, almost no increase in the serum cortisol levels was observed after 30 minutes and 60 minutes following ACTH administration, thus suggesting the presence of adrenal insufficiency. According to a contrast-enhanced CT of the abdomen, atrophy of both adrenal glands was observed; however, there was no macroadenoma or the like according to the contrast-enhanced MRI of the brain. Based on the results of the ACTH continuous load test, triple anterior pituitary stimulation test with corticotropin-releasing hormone (CRH)<i>,</i> thyrotropin-releasing hormone (TRH), and gonadotropin-releasing hormone (GnRH),<i>&#xa0;</i>growth hormone-releasing peptide-2 (GHRP-2) load test, and insulin hypoglycemic load test, no abnormalities were found in his pituitary functions other than ACTH-cortisol, and no history of trauma or adenoma lesions, leading to a diagnosis of a sudden isolated ACTH&#xa0;deficiency. The patient has remained stable with no hypoglycemic episodes since treatment with hydrocortisone was initiated. Based on the fact that isolated ACTH deficiency is often associated with a complication of autoimmune endocrine disease, an autoimmune mechanism has been speculated. Although the disease is rare, it is an important disease that affects the quality of life (QOL) of patients, and it should therefore be considered when making a differential diagnosis.