Endocrine Abstracts

Background: The 250microgram synacthen test may give false negative responses, and will not allow easy discrimination between primary and secondary dysfunction. Furthermore, the use of synacthen is contra-indicated on the data sheet in certain common conditions such as asthma, and may not easily be interpreted in women taking oral oestrogen. Some patients with incipient adrenal failure may have elevated plasma ACTH, whilst passing the synacthen test. We have routinely measured basal ACTH during synacthen testing and describe here the utility of this measurement.

Subjects and methods: The case notes of 106 patients undergoing a paired basal early morning ACTH and Synacthen testing were reviewed: i) normal HPA axis (30min cortisol response to Synacthen >550nmol/L), n=64; ii) primary adrenal insufficiency (PAI), n=4; iii) secondary adrenal insufficiency (SAI), n=8; iv) pituitary disease with a normal response n=20; v) patients taking oral oestrogen, n=10.

Results: Basal 0900h plasma ACTH did not correlate significantly with basal total cortisol (F0), 30min cortisol (F30) or the increment in cortisol in any group. However, F0 strongly correlated with F30 in the oestrogen group (r=0.82, p<0.001), but not in other groups. Analysis demonstrates that whilst basal ACTH and F0/ACTH ratio will discriminate all cases of PAI, these parameters alone do not allow discrimination between normal HPA axis and SAI groups. However, patients with SAI can be separated from the normal HPA axis group based on a F0 <215nmol/l (sensitivity 100%, specificity 98.5%).

Conclusions: The reason for the relationship between the basal and stimulated cortisol in women receiving oral oestrogen is unexplained, but may result from a smoothing effect of CBG, and merits further study. Basal plasma cortisol and the F0/ACTH ratio may allow discrimination between groups, and may be a useful test in those people in whom synacthen testing is contra-indicated.