Which type 2 diabetes mellitus patients should be screened for subclinical Cushing's syndrome?

Diabetes or impaired glucose tolerance are common in patients with Cushing's syndrome (CS). In this study we investigated the incidence of subclinical Cushing's syndrome (SCS) in type 2 diabetic patients with poor and good metabolic control and the relationship between blood glucose and cortisol levels.

A total of 400 type 2 diabetes mellitus (T2DM) patients (Group A, HbA1c ≥8%, n=250; Group B, HbA1c ≤6.5%, n=150) were included in the study. Biochemical tests followed by the 1 mg dexamethasone suppression test (DST) were performed. If post-DST cortisol levels were above 1.8 μg/dl (non-suppressed group), the 2 mg DST test was performed for 2 days. Among the patients, post-DST cortisol of at least 1.8 μg/dL and midnight serum cortisol levels of at least 7.5 μg/dL confirmed the diagnosis of CS.

While SCS was observed in 5 patients (2%) in Group A, no case was observed in group B. There was a statistically significant difference between groups for basal cortisol and post-DST cortisol results (p<0.001 and p<0.001, respectively). Microvascular complication rates were higher in the non-suppressed group (p=0.007). Post-DST cortisol levels had a positive correlation with the number of complications and HbA1c levels (respectively r=0.213, p<0.001, and r=0.191, p<0.001). Multivariate regression analysis revealed that durations of DM, HbA1c level, and post 1 mg DST cortisol levels were associated with the number of complications.

This study shows the presence of SCS in a notable number in T2DM patients having poor metabolic control. Screening with 1 mg and 2 mg DST of T2DM patients with poor metabolic control who are also obese and hypertensive and have microvascular complications may be an appropriate method for detection of SCS.