Synopsis
Choose stimulation or suppression testing for a defined endocrine question, prepare the patient and assay correctly, supervise predictable hazards, and interpret the complete response rather than one number.
- A stimulation test asks whether an axis with possible deficiency can respond; a suppression test asks whether suspected autonomous secretion can be switched off.
- Write the clinical question before booking: the same test can be inappropriate when pre-test probability, recent treatment or acute illness changes.
- Use the endocrine unit's current protocol because dose, sampling times, assay-specific thresholds, contraindications and rescue arrangements differ between laboratories.
- For the licensed adult 30-minute short Synacthen test, give tetracosactide 250 micrograms once by intramuscular or intravenous injection and measure cortisol immediately before and exactly 30 minutes after; use the laboratory's current assay-specific interpretation.
- A standard overnight dexamethasone suppression protocol gives dexamethasone 1 mg orally at 23:00 and measures serum cortisol at 09:00; check adherence, interactions, oestrogen exposure, alcohol, sleep and the local assay decision limit before interpreting non-suppression.
- Oral glucose should suppress growth hormone in normal physiology; failure of suppression supports acromegaly after a raised age-adjusted IGF-1 and clinical assessment.
- Water-deprivation testing can cause dangerous dehydration or hypernatraemia and belongs under experienced supervision after simpler confirmation of hypotonic polyuria.
- A supervised fast or mixed-meal study investigates documented hypoglycaemia mechanisms; symptoms without a low laboratory glucose do not justify an indiscriminate prolonged fast.
- Record the exact stimulus or suppressor dose, sample times, achieved physiological stimulus, assay and laboratory decision limit; do not transfer cortisol, growth-hormone or copeptin cut-offs between methods without validation.
Key red flags
Compatible symptoms during a low laboratory plasma glucose that resolve with correction establish the clinical event and can justify a supervised fast or meal test selected by timing.
Acute illness, pregnancy-specific risk, severe cardiac or seizure disease, uncontrolled electrolyte disturbance or inability to consent may contraindicate or modify provocative testing and requires senior review.
Investigation priorities
Assess adrenal cortisol secretory reserve when basal morning assessment remains indeterminate or specialist confirmation is needed.
Management branches
Basal tests and phenotype leave a clinically important endocrine diagnosis unresolved.
- Define whether the suspected abnormality is deficient reserve or autonomous excess, and quantify pre-test probability using phenotype, paired basal hormones and medicines.
- Choose the validated stimulation or suppression test whose result will change treatment, referral or imaging, confirming that a simpler repeat sample cannot answer it.
- Review contraindications, pregnancy, acute illness, organ function and all interfering prescriptions or supplements with endocrinology and biochemistry.
- Book the correct supervised setting, consent process, timed sampling and rescue plan, and name the clinician who will interpret and act on the complete test.
Key medicines
Tetracosactide (Synacthen)For the licensed adult 30-minute test, give tetracosactide 250 micrograms once by intramuscular or intravenous injection after the baseline cortisol sample, then collect cortisol exactly 30 minutes later.Rare hypersensitivity can occur; ensure resuscitation arrangements. Recent central ACTH loss may produce a misleading preserved response, and thresholds differ with cortisol assay and sampling protocol.
Dexamethasone for suppression testingFor a standard adult overnight suppression test, give dexamethasone 1 mg orally at 23:00 and obtain serum cortisol at 09:00 the next morning, using the endocrine laboratory's validated interpretation.Non-adherence, malabsorption, CYP3A inducers or inhibitors, oestrogen-related binding changes and acute stress can distort interpretation. This diagnostic dose is not treatment for adrenal crisis.