Patients on long term prednisolone therapy have suppression of their hypothalamo-pituitary-adrenal (HPA) axis. The short synacthen test (SST) is routinely employed to assess the viability of the HPA axis. However, as this test examines only adrenal function it may fail to identify those patients with secondary adrenal insufficiency. The overnight metyrapone test (OMT) may be a better test as it examines the whole HPA axis.
We have compared the responses to the two tests in 18 control subjects and 18 patients who had been taking prednisolone for inflammatory conditions for over 3 months. Each subject was given, either 0.25mg tetracosactrin acetate at 9am with bloods taken at 0, 30 and 60 minutes for cortisol measurement, or metyrapone (30mg/kg) at 11pm with blood for ACTH, 11-deoxycortisol and cortisol at 9am the next morning. This study was approved by the St Thomas' Hospital ethics committee.
Compared with controls, patients had significantly lower(p less than 0.001)mean cortisol at 0, 30 and 60 minutes and cortisol increment after SST. They also had significantly lower (p less than 0.001) mean ACTH, cortisol, 11-deoxycortisol and total serum glucocorticoids after MOT. There was a positive correlation between the SST and OMT responses (r=0.83). However, there was a 22% discordance between the 30 minute cortisol response after SST and the total glucocorticoid (cortisol plus 11-deoxycortisol) response after OMT. The SST identified fewer patients with HPA insufficiency. Our data also show that the total serum glucocorticoid is a better indicator of adrenal insufficiency after OMT than 11-deoxycortisol alone. Our results suggest that the glucocortcoid response to OMT is more sensitive than the SST in diagnosing insufficiency of the HPA axis.