ACTH-secreting pituitary adenomas: size does not correlate with hormonal activity

ACTH-secreting pituitary adenomas (Cushing's disease, CD) are the most frequent cause of Cushing's syndrome. To test whether the size of ACTH-secreting adenomas correlates with the degree of biochemical and clinical features of hypercortisolism, we retrospectively reviewed all newly diagnosed CD patients seen at our institution by two neuro-endocrinologists over a 10-year time period. We documented the number of clinical manifestations and baseline hormonal measurements. There were 37 microadenomas (μAs) and 16 macroadenomas (MAs). We sought to characterize the relationship between tumor size (μA vs. MA) and number of signs and symptoms of hypercortisolism and biochemical assessment of hypercortisolemia. There were no significant differences in mean age, BMI, or prevalence of hypertension and type 2 diabetes between the μA and MA groups. However, the MAs had fewer clinical manifestations of hypercortisolism (29.4% vs. 36.1%, P = 0.02) compared to μAs. There was a higher prevalence of easy bruisability and proximal muscle weakness in the μAs, but otherwise the prevalence of signs and symptoms were similar between groups. The MAs had a lower random serum cortisol (18.2 ± 2.4 vs. 25.9 ± 1.8 mcg/dl, P = 0.018), lower cortisol:ACTH ratio (0.25 ± 0.03 vs. 0.42 ± 0.05, P < 0.048), and lower cortisol:tumor diameter ratio (14.1 ± 2.2 vs. 56.8 ± 7.2, P < 0.0001) than the μAs. We conclude that tumor size does not directly correlate with the extent of hormonal activity in ACTH-secreting adenomas. Biochemical activity and clinical manifestations may be mild even in larger tumors, and therefore a high index of suspicion may be necessary to recognize hypercortisolism in pituitary MAs.