Adrenal Hyperfunction - Cushing Syndrome

Indications for Testing

• Resistant diabetes, refractory hypertension, or multiple signs/symptoms of Cushing syndrome after ruling out metabolic syndrome or polycystic ovarian syndrome (PCOS)
• Do not test patients who have hypercortisolism secondary to administration of glucocorticoids (oral, inhaled, injections, topical)

Laboratory Testing

• Initial testing (Endocrine Society, 2008; NCCN, 2016)
  •  Salivary cortisol (collected between 2300 hours and midnight) – at least 2 measurements
    • Reflects circadian nadir so elevations are more relevant
    • Salivary samples are stable over time
    • Counsel individual to collect on quiet, restful evening
    • Cutoff values are test and age specific
  • 24-hour urine free cortisol (UFC) – at least 2 measurements
    • 24-hour urine collection – random measurements not recommended
    • Urinary cortisol levels >3 times the upper limit of normal are unequivocal and confirm diagnosis
    • Results not affected by taking oral estrogen
  • Dexamethasone suppression testing (DST) – overnight and 2-day tests are available
    • Does not interfere with cortisol tests; other glucocorticoids will influence cortisol levels and must be discontinued prior to testing
      • Other hormones that may interfere with cortisol testing – exogenous testosterone, Nature-Throid, estrogens (eg, birth control), megestrol acetate
      • Estrogen-containing drugs should be withdrawn for 6 weeks prior to testing
    • Low-dose (1 mg) DST
      • Patient should take 1 mg dexamethasone between 2300 hours and 2400 hours
      • Goal – should suppress adrenocorticotropic hormone (ACTH) production if hypothalamic-pituitary-adrenal axis (HPA) is normal
      • Serum or urine should be collected between 0800 and 0900 hours and tested for free cortisol
        • ACTH collected at same time may be helpful in case of positive test
      • Dexamethasone is measured to confirm administration and normal metabolism
    • 2-day, 2 mg test – may be used to confirm that ACTH can be suppressed if overnight, low-dose test is equivocal
      • Administer 0.5 mg every 6 hours starting at 0800 for 2 days for a total of 8 doses (2 mg)
      • Collect serum for cortisol 2-6 hours after last dose administered
      • Collecting ACTH at same time will assist with etiology in case of positive test
      • Collect dexamethasone to confirm administration and normal metabolism
      • Urinary corticosteroid excretion on day 2 of dexamethasone administration has been used (compare to baseline day urinary corticosteroid excretion)
        • Not currently recommended, but literature often cites concentrations
    • To confirm elevated results – repeat either of the DST tests or the cortisol tests
      • If high suspicion remains – use high-dose dexamethasone (8 mg) overnight test
        • Similar protocol to low-dose overnight dexamethasone suppression test
        • Negative suppression testing – suggests etiology other than Cushing syndrome
• Follow-up testing (Endocrine Society, 2008; NCCN, 2016)
  • Endocrinology referral
  • Serum ACTH in conjunction with suppression tests – aid in determining whether disease is adrenal, pituitary, or ectopic ACTH-based
    • If ACTH is suppressed (<10 pg/mL) – suggests etiology is ACTH independent and pathology is adrenal (adenoma, carcinoma)
      • Refer to Imaging Studies section
    • If ACTH is >20 pg/mL (high levels) – suggests ectopic ACTH
      • Refer to Imaging Studies section
    • If ACTH is intermediate between these levels – likely Cushing syndrome and magnetic resonance imaging (MRI) should be done to visualize tumor
      • ACTH-producing tumors are often not visualized (microadenoma)
      • Bilateral inferior petrosal sinus sampling is indicated (Pappachan, 2017)
        • Note: this is technically difficult and should only be performed in high-volume center
          • Petrosal ACTH to serum ACTH ratio is measured before and after corticotropin stimulation
          • Absence of change in gradient – suggestive of ectopic ACTH-producing tumor
          • Significant increase in pre- and poststimulation – expected in Cushing syndrome

Histology

• Pathology review of specimen (biopsy tumor) to classify type of tumor
  • Immunohistochemistry stain – ACTH stain most common

Imaging Studies

• MRI pituitary with inferior bilateral petrosal sampling of ACTH to differentiate between Cushing syndrome and ectopic source (if necessary)
  • If ACTH is low – MRI of adrenal glands
  • If ACTH >20 pg/mL – search for ectopic ACTH-secreting tumor
• CT adrenal protocol or MRI adrenals – to identify unilateral/bilateral tumor or hyperplasia

Prognosis

• Untreated Cushing syndrome – associated with excess morbidity/mortality secondary to cardiovascular disease
• If tumor is benign and removed – mortality associated with comorbid diseases developed in association with tumor may normalize
• Malignant tumors producing ACTH have a poor prognosis

Differential Diagnosis

• Obesity/metabolic syndrome
• Type 2 diabetes mellitus
• PCOS
• Depression
• Hypertension
• Malignancy associated with ectopic ACTH production – most common are small cell lung cancer, thymoma, medullary carcinoma of the thyroid