Endocrine Abstracts

Background: Cushing’s disease (CD) may be associated with equivocal results on biochemical investigations.

Aim: To evaluate the usefulness of dexamethazone suppression tests in the diagnostic work-up of CD.

Patients and methods: Seventy patients with CD [median age 38 yrs(16–76), 53 females] presenting between 1976–2005 were studied. 24-hr urinary free cortisol (UFC), overnight (oDST) (1 mg at 23:00 h), low dose (LDDST) (2 mg daily over 2 days), high dose (HDDST) (8 mg daily over 2 days) dexamethasone suppression tests were assessed. Statistical analyses were based on subjects with available data on each occasion.

Results: At initial evaluation 96.4%(53/55) had UFC above normal [median 674 nmol/24 hr(195–5220)]. Failure of suppression of serum cortisol (<50 nmol/l) was found in 0%(0/30) on the oDST [median 465 nmol/l(151–1396)] and 97.7%(42/43) on the LDDST [median 346 nmol/l(49–899)]. 29.2%(14/48) suppressed serum cortisol <50% on the HDDST [median 81.7%(0–92.7%)]. There was no difference in the UFC among subjects suppressing serum cortisol >50% or <50% on HDDST (median 597 vs 1620 nmol/24 hr, P=0.2). There was significant correlation between UFC and serum cortisol on oDST (r=0.6, P=0.04), serum cortisol on LDDST (r=0.5, P=0.03), but not percentage fall of serum cortisol on HDDST (P=0.09). No significant correlation between percentage fall of serum cortisol on the HDDST and serum cortisol values on the oDST (P=0.1) or on the LDDST (P=0.1) was found. There was a significant correlation between the percentage fall of serum cortisol on the LDDST and the HDDST (48hours) (r=0.4, P=0.03).

Conclusions: In our series, the LDDST was overall satisfactory for the diagnosis of hypercortisolism. The HDDST showed high rate of false negative results rendering it unsatisfactory in the diagnostic work-up of CD. The correlation between percentage fall of serum cortisol on the LDDST and HDDST suggests that the LDDST may be useful in predicting the pituitary origin of Cushing’s syndrome.