: A rare case with ectopic adrenocorticotrophic hormone syndrome (EAS) caused by medullar thyroid carcinoma (MTC) in mediastinum was reported. This 49 year-old male patient initially presented with serious and intractable hypokalemia. Endocrine evaluations showed increased levels of adrenocorticotrophic hormone (ACTH) and urinary free cortisol, which could not be suppressed more than 50% by high-dose dexamethasone suppression test. Computed tomography (CT) scan detected a 5×5×5 cm mass at the bottom of thyroid in anterior mediastinum. The patient underwent total thyroidectomy with central compartment and ipsilateral modified radical neck dissection. Pathological examination showed an infiltrating thyroid medullary carcinoma with abundant amyloid deposition, meanwhile immunohistochemical positive for ACTH. After surgery, serum levels of kalium, as well as cortisol and ACTH returned to normal range. During follow-up, the patient's clinical manifestation of Cushing syndrome relieved.