Ectopic production of ACTH and corticotropin-releasing hormone (CRH)

Abstract

The most common ectopic production of a pituitary hormone is the one of ACTH leading to Cushing's syndrome. Ectopic ACTH-hypersecretion is the cause of Cushing's syndrome in 10–15% of all cases. The ACTH-secreting tumours are often oat-cell carcinomas of the lung, less frequently pancreatic cancers, hypernephromas, or C-cell carcinomas of the thyroid. Some of these tumours may be benign or semi-benign as the rare carcinoid tumours and cause great problems in the differential diagnosis of ACTH-dependent hypercortisolism. Out of 173 of our patients with Cushing's syndrome observed in the last 12 years 21 were caused by ectopic ACTH-production. Of these 21 patients 13 have a small cell carcinoma of the lung. The ectopic ACTH-syndrome often has typical clinical features caused by the levels of ACTH and cortisol leading to hypocalaemic alcalosis with muscle weakness and wasting, carbohydrate intolerance, and hypertension with oedema. The survival time in many of these patients is not long enough to allow them to develop typical signs of Cushing's syndrome though they are often highly pigmented. These patients are easily diagnosed. However, patients with small tumours which do not cause very elevated ACTH-levels and who have the more typical clinical signs of full-blown Cushing's syndrome are difficult to recognize. For the differential diagnosis of ACTH-dependent Cushing's syndrome the corticotropin-releasing hormone (CRH) stimulation test and dexamethasone suppression test with high doses are helpful. In special cases the venous sampling procedure for ACTH-measurements is necessary, also CT or NMR is helpful. Ectopic CRH-production is a rare cause of ACTH-dependent Cushing's syndrome. Patients with ectopic CRH-production and consecutive ACTH-hypersecretion from the pituitary have not been studied extensively. There are especially no well documented results of the use of the CRH-stimulation test in vivo in this group of patients with Cushing's syndrome. On the other hand, in the documented cases, not only CRH-, but also ACTH-production was found in the tumours. So far, this rare cause of ACTH-dependent Cushing's syndrome has to be excluded or confirmed by the measurement of endogenous CRH-levels. But until now we have not been able to detect one single case of ectopic CRH-production using a sensitive homologous CRH-radioimmunoassay over a period of more than 8 years in which we have seen nearly 120 newly diagnosed patients with ACTH-dependent Cushing's syndrome. Only in the plasma and tumour tissue of two patients of other groups have we found high CRH-levels.