Assessment of the effect of single dose dexamethasone on the hypothalamic-pituitary axis

Our adrenal glands produce cortisol which is our body’s natural “stress hormone”. Cortisol production is controlled by the hypothalamus and pituitary glands in the brain (hypothalamic-pituitary adrenal axis). This involves a feedback loop to prevent over- or under-production of cortisol. Dexamethasone (a synthetic form of cortisol) has been shown to significantly reduce postoperative nausea and vomiting. Currently most patients having a general anaesthetic receive a single 8mg dose of dexamethasone. This has been studied in relation to a number of different surgery types and in adults appears to be safe and effective. Dexamethasone has a long duration of action, typically quoted to be 36-72hours although the plasma half-life of dexamethasone is shorter at approximately 6 hours. Dexamethasone is used by endocrinologists to assess whether there is cortisol overproduction i.e. “dexamethasone suppression test” as it doesn’t interfere with the laboratory measurement of cortisol. Similar to the overnight dexamethasone suppression test it has been shown that after a single 8mg dose cortisol suppression occurs. What isn't known is how long the cortisol suppression lasts and whether other pituitary hormone levels are also suppressed.