Endocrine Abstracts

Aim: We previously reported preliminary evidence that use of TRH with CRH during inferior petrosal sinus sampling (IPSS) in Cushing’s syndrome gave additional information about quality of pituitary venous sampling and significance of gradients. We now report long-term experience of this technique in 40 patients investigated 1993–2005. We used standard central:peripheral ratios (C:P) of 2:1 basally and 3:1 after stimulation to indicate pituitary secretion and intersinus gradient (ISG) of >1.5:1 to indicate lateralisation of ACTH secretion or TSH/PRL after TRH. We attempted to mathematically ‘correct’ the levels and gradients of ACTH for presumed unequal sampling of pituitary blood evidenced by TSH/PRL ISGs.

Results: 37 had results consistent with Cushing’s disease: 33 patients had basal ACTH C:P > 2 and peak C: P >3; three had basal ACTH C:P <2 but peak C:P >3 after CRH.; one patient had C: P >2 basally but <3 after CRH peak. 3 had no significant C:P gradient consistent with ectopic ACTH secretion but TSH/PRL ISG indicated the possibility of inadequate pituitary sampling on one side in 2 cases.

Only 9 patients (22.5%) had equal pituitary venous sampling confirmed with maximum TSH/PRL ISG <1.5; ACTH ISG was >1.5 in 6 of these and ISG did not change significantly after ‘correction’ in any patient.

31 patients (77.5%) had unequal sampling with maximum TSH/PRL ISG >1.5; in 10 cases ACTH ISG did not change even with ‘correction’ but in the remainder the conclusion regarding presence of absence of ACTH lateralisation was significantly altered by ‘correction’ for unequal sampling.

Conclusion: IPSS involves unequal sampling of pituitary venous blood in a majority of cases as evidenced by TSH/PRL ISG before and after TRH. Correction of ACTH ISG using TSH/PRL ISG alters the conclusion on presence or absence of ACTH lateralisation in 52.5% of cases and we believe increases accuracy.