Differential Diagnosis for Morning Cortisol of 28 with Morning Dexamethasone

Given a morning cortisol level of 28 with morning dexamethasone, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis

  • Cushing's syndrome: This condition is characterized by excess cortisol production. The elevated morning cortisol level despite dexamethasone administration (which should suppress cortisol production in normal individuals) strongly suggests Cushing's syndrome. The dexamethasone suppression test is a key diagnostic tool for Cushing's syndrome, and failure to suppress cortisol levels indicates autonomous cortisol production.

• Other Likely Diagnoses

  • Adrenal incidentaloma: An adrenal incidentaloma is a tumor discovered incidentally on imaging performed for other reasons. Some of these tumors can produce cortisol, leading to Cushing's syndrome. The lack of suppression with dexamethasone could indicate an adrenal source of excess cortisol.
  • Familial Cushing's syndrome: Although rare, familial forms of Cushing's syndrome can occur due to genetic mutations affecting the regulation of cortisol production. These cases might not suppress with dexamethasone.
  • Exogenous steroid use: The use of exogenous steroids can suppress the body's natural production of cortisol, but certain steroids might not be fully suppressed by dexamethasone, or the patient might not have disclosed steroid use.

• Do Not Miss Diagnoses

  • Pheochromocytoma: Although primarily known for secreting catecholamines, some pheochromocytomas can co-secrete cortisol or have associated cortisol-producing adrenal nodules. Missing this diagnosis could be catastrophic due to the potential for severe hypertension and other complications.
  • Adrenal carcinoma: A rare but aggressive tumor that can produce cortisol, leading to Cushing's syndrome. The failure to suppress cortisol with dexamethasone could indicate such a tumor.

• Rare Diagnoses

  • McCune-Albright syndrome: A rare genetic disorder that can lead to Cushing's syndrome among other endocrine abnormalities. It is characterized by café-au-lait spots, polyostotic fibrous dysplasia, and various endocrine disorders.
  • Primary pigmented nodular adrenocortical disease (PPNAD): A rare cause of Cushing's syndrome, often associated with Carney complex, characterized by multiple small nodules in the adrenal glands that produce cortisol autonomously.