From the Guidelines
Yes, I would recommend proceeding with a pituitary MRI despite the normal ACTH level. The abnormal dexamethasone suppression test (DST) with a cortisol level of 2.1 and dexamethasone level of 397 suggests possible Cushing's syndrome that isn't fully suppressing with dexamethasone as expected. While the ACTH level of 17 falls within the normal range, this doesn't rule out a pituitary source (Cushing's disease) [ 1 ]. ACTH levels in Cushing's disease can sometimes present within normal ranges, especially if there's cyclical cortisol production. Since the abdominal CT showed no adrenal abnormality, which rules out adrenal adenoma or carcinoma, the next logical step is to evaluate the pituitary gland as a potential source of the abnormal cortisol regulation.
A pituitary MRI with contrast would be the appropriate next test to look for microadenomas or other pituitary abnormalities [ 1 ]. The use of 3T MRI is recommended over 1.5T MRI where available, as it provides higher resolution and better detection of small lesions [ 1 ]. If the MRI is negative, further testing such as bilateral inferior petrosal sinus sampling might be considered to definitively determine if there is a pituitary source for the abnormal cortisol production.
Some key points to consider:
- The size of the tumor does not necessarily correlate with the degree of hypercortisolism in Cushing's disease [ 1 ].
- Positron emission tomography (PET) has been explored as an alternative to, or in combination with, MRI for localization of corticotroph adenomas, but its use is not widely available and requires further study [ 1 ].
- The abnormal DST result and lack of adrenal abnormality on CT scan justify proceeding with pituitary MRI, despite the normal ACTH level [ 1 ].
From the Research
Abnormal DST Results and Next Steps
- The patient's cortisol level was 2.1 with a Dexamethasone level of 397, indicating an abnormal DST result 2.
- The baseline ACTH level was 17, which is within the normal range.
- An abdominal CT with and without contrast showed no adrenal abnormality.
Considerations for Pituitary MRI
- Despite the normal ACTH level, the abnormal DST result suggests the possibility of Cushing's syndrome or another adrenal disorder 3, 4.
- The study by 4 found that the presence of ≥ 2 among UFC>70 μg/24 h, ACTH<10 pg/ml, and 1 mg-DST>3.0 μg/dl had the best accuracy in predicting the improvement of ≥ 2 metabolic endpoints after surgery.
- However, the patient's ACTH level is within the normal range, and the cortisol level is not significantly elevated.
- The study by 2 suggests that post-DST free cortisol measurements can add value to the diagnostic workup, particularly in patients with optimal dexamethasone concentrations and post-DST total cortisol between 1.8 and 5 mcg/dL.
Decision to Proceed with Pituitary MRI
- Given the abnormal DST result and the lack of adrenal abnormality on CT, a pituitary MRI may be considered to rule out a pituitary source of ACTH-dependent Cushing's syndrome 5.
- However, the decision to proceed with a pituitary MRI should be based on a comprehensive evaluation of the patient's clinical presentation, laboratory results, and medical history.