Inferior petrosal sinus sampling (IPSS) is the investigation to look for the source of ACTH in cases with pituitary adenomas <6–9 mm or in those with no adenoma/equivocal pituitary imaging.40 IPSS is based on the measurement of ACTH from both the right and left petrosal sinuses in comparison to the peripheral vein. This evidence concerns the gene POMC and adenoma.