Time HPA axis testing after glucocorticoid dose
STEP 1 — Gather Information
Confirm patient is on glucocorticoid therapy; record the type (hydrocortisone vs synthetic glucocorticoid) and timing of the last dose.
STEP 2 — Rule In / Rule Out
If last dose was hydrocortisone, proceed only if ≥18–24 hours have elapsed; if synthetic glucocorticoid, wait longer (e.g., ≥24 hours) before testing.
STEP 3 — Classify or Stratify
Stratify by glucocorticoid type: hydrocortisone requires 18–24h washout; synthetic glucocorticoids require extended washout (consider 24h or more based on agent potency).
STEP 4 — Decide
Schedule the HPA axis biochemical test (e.g., morning cortisol or ACTH stimulation) after the appropriate washout period has been met.
Clinical Guardrails / Mimics / Pitfalls
Do not test sooner than recommended as glucocorticoid suppression can falsely normalize results; consider estrogen-induced CBG elevation that may raise total cortisol; avoid testing during acute illness; random cortisol is unreliable for this purpose.
Concrete Clinical Example
A patient on hydrocortisone 15 mg morning and 5 mg evening requests an 8 AM cortisol test; last dose was 10 PM prior night. Since only 10 hours have elapsed, defer testing until at least 2 PM the same day (18h post-dose) to avoid false-normal result.
Source: Hormonal Replacement in Hypopituitarism in Adults: An Endocrine Society Clinical Practice Guideline, Endocrine Society, 2016, DOI:10.1210/jc.2016-2118