Identification of source of excessive ACTH production by improved selective venous sampling: simultaneous assay of PRL as a pituitary marker.

To identify an obscure site of excessive ACTH production, selective venous sampling was performed by Seldinger's method in a 32-year-old man. Differential diagnosis between an ectopic ACTH producing tumor and Nelson's syndrome was quite inconclusive in this patient in spite of extensive clinical and laboratory evaluations, including pituitary surgery. In this study, PRL was measured in every sample as a reliable marker for pituitary drainage. Significantly higher rations (2.6 to 5.3) of catheter to peripheral (C/P) concentration of PRL at sites closer to the pituitary indicated successful samplings of pituitary drainage. C/P ratios of ACTH correlated very closely (correlation coefficient = 0.906, p less than 0.01) with those of PRL at every sampling site. Thus, it was concluded that the site of excessive ACTH production in this patient was in the pituitary gland. The methods applied in this study appear to be useful in differentiating ectopic from eutopic production of other pituitary hormones as well.