Schmidt syndrome is a polyendocrinopathy characterized by multiple organ failures. Patients with two or more of the following characteristics are diagnosed with this syndrome: Graves' disease, adrenal insufficiency, autoimmune thyroiditis, and type I diabetes. The present case report concerns a Schmidt syndrome patient (polyglandular autoimmune syndrome type II). The patient is a 42-year-old woman with type I diabetes and complaints of imbalance, weakness and lethargy, weight loss, darkened skin, blood pressure drop, hypoglycemia, and hyperkalemia who visited the Hospital frequently and was discharged every time after intravenous potassium chloride and dextrose injections and orders to reduce insulin consumption at home. Her blood cortisol level was measured in her last visit with suspicion of adrenal insufficiency, which was revealed to be low (less than 0.5 μg/dL). A high ACTH, low aldosterone, and normal renin were also reported, indicating secondary adrenal insufficiency. Furthermore, T4 and TSH tests were performed, reporting a TSH lower than 0.2 in the initial TSH test, which was revealed to be normal after the test was repeated. Subclinical hyperthyroidism was indicated, given the high TRAb figure. The patient underwent treatment with fludrocortisone and prednisolone, which improved her condition. This report accentuates that diabetic patients with reduced insulin requirements must be checked for adrenal insufficiency immediately to prevent potential complications.