We report a case of a young woman with Cushing's syndrome (CS), in whom although endocrine investigations and negative pituitary imaging were suggestive of ectopic ACTH secretion, the results of inferior petrosal sinus (IPS) sampling after coricotropin-releasing hormone (CRH) stimulation were suggestive of pituitary ACTH hypersecretion.111 In-labelled octreotide and high-resolution computer tomography (CT) revealed a lesion possibly responsible for the ACTH source in the thymus. Thymectomy confirmed concomitant ectopic CRH and probable ACTH production by a thymic neuroendocrine carcinoma. After an 8-year remission period the patient developed a clinical and biochemical relapse. A high-resolution computed tomography (CT) scan of the thorax showed a 2-cm nodule in the thymic bed, which was positive on a [ Case reportA 25-year-old woman initially presented with recentonset depression, hirsuitism, acne and bruising suggestive of Cushing's syndrome (CS); clinical examination revealed arterial hypertension (170/120 mmHg) and mild proximal myopathy. Subsequent investigations showed hypokalemic alkalosis (serum K þ , 2.3 mmol/l (normal range 3.5 -5.0 mmol/l); HCO 3 , 32 mmol/l (normal levels 24^2 mmol/l)), marked hyperandrogenemia (testosterone, 12.5 nmol/l (normal range 0.5 -2.5 nmol/l)), normal fasting glucose concentrations and autonomous hypercortisolemia (elevated 24-h urinary free cortisol (UFC) of 4064 nmol per 24 h (normal range 20 -300 nmol per 24 h), serum midnight cortisol of 1161 nmol/l and inadequate cortisol suppression to a formal low-dose dexamethasone suppression test).An ectopic source of adrenocorticotropic hormone (ACTH) secretion was suggested by a marked elevated plasma ACTH concentration (1350 ng/l (normal values , 25 ng/l)), and a failure to suppress serum cortisol following a high-dose dexamethasone suppression test (HDDST). Inferior petrosal sinus (IPS) sampling after corticotropin-releasing hormone (CRH) stimulation, however, was more in keeping with Cushing's disease than with ectopic ACTH production as it showed a brisk ACTH increment following CRH administration (Table 1) (Fig. 1A). A further high-resolution CT showed a 2-cm thymic nodule in the upper mediastinum. Whole-body catheterization and sampling did not reveal an ectopic ACTH source. However, a greater than 3:1 CRH gradient suggestive of ectopic CRH production between the left brachiocephalic vein (BCV) and superior vena cava (SVC) and their simultaneous peripheral samples (BCV, 90 pg/ml and SVC, 100 pg/ml vs 30 pg/ml respectively) was shown. Based on the above findings a working diagnosis of a neuroendpcrine tumor of the thymus (thymic carcinoid tumor) secreting CRH was made.The patient underwent a mediastinotomy and thymectomy. A central firm nodule within the thymus