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High-dose dexamethasone suppression test

ISSN 2398-2950

Synonym(s): HDDST


  • A high dose of dexamethasone inhibits pituitary ACTH secretion through negative feedback and hence this decreases plasma [cortisol] Endocrinology: hypopituitary adrenal axis - diagram .


In combination

  • To differentiate adrenal-dependent hyperadrenocorticism (ADH) from pituitary-dependent hyperadrenocorticism (PDH) after hyperadrenocorticism Hyperadrenocorticism has been diagnosed by screening test .

Other points

  • Marked individual variation in timing of suppression necessitates multiple post-suppresssion sampling.


Source of test material

Quantity of test material

  • 1-2 ml whole blood (1ml plasma or serum ) per sample.

Sample collection technique

  • Fast animal for 12 hours.
  • Collect 1-2 ml heparinized/clotted blood.
  • Inject dexamethasone Dexamethasone 1.0mg/kg IV.
  • Collect 1-2 ml heparinized/clotted blood at 4 and 8 hours.
  • Separate plasma/serum.
  • Perform cortisol assay Blood biochemistry: cortisol on all samples.

Quality control

Timing of test

  • Alternatively can be combined with ACTH test.
    • Collect basal blood sample.
    • Inject 0.1 mg/kg dexamethasone IV.
    • Collect second blood sample at 2 hours.
    • Immediately inject 0.125 mg synthetic ACTH IV.
    • Collect blood sample 1 hour after ACTH administration.
    • Perform cortisol assay Blood biochemistry: cortisol on all samples.

Sample storage

  • Once separated, plasma/serum can be frozen or stored in refrigerator for up to one week.

Sample transport

  • Normal mail.



  • [Cortisol] available at many commercial laboratories.



  • Good for identifying pituitary tumors.


  • Some pituitary tumors (macroadenomas?) respond like adrenal tumors.

Technique intrinsic limitations

  • [Plasma cortisol] does not suppress in approximately 50% of PDH cases probably because ACTH is released from the pars intermedia rather than the pars distalis and the pars intermedia is less sensitive to the negative feedback of high [plasma cortisol].
  • Does not differentiate adrenocortical adenomas from adrenocortical carcinomas.

Technician extrinsic limitations

  • Must be able to give intravenous injection of dexamethasone and collect subsequent blood samples without stressing animal.

Result Data

Normal (reference) values

  • Normal: post-dexamethasone [cortisol] at 4 and 8 hours <50% of resting [cortisol] (and normal stimulation to >400 nmol/l after ACTH).

Abnormal values

  • PDH Hyperadrenocorticism (pituitary neoplasia): post-dexamethasone [cortisol] in cats with confirmed hyperadrenocorticism at 4 or 8 hours <50% of resting [cortisol].
  • Failure of dexamethasone to suppress [cortisol] requires further investigation eg ACTH assay ACTH assay to distinguish PDH from ADH.
  • Cats with hyperadrenocorticism show little cortisol suppression after dexamethasone and an exaggerated response after ACTH administration in combined test).

Further Reading


Refereed papers

  • Recent references from VetMedResource and PubMed.
  • Boland L A & Barrs V R (2017) Peculiarities of feline hyperadrenocorticism - Update on diagnosis and treatment. J Feline Med Surg 19, 933-947 PubMed.