Dynamic endocrine testing

01Purpose and principlesWhat the assessment is for and the core concepts behind it.

Basal hormones may be inconclusive because secretion is pulsatile, circadian or stress-responsive. Dynamic tests apply a controlled physiological challenge and measure whether the expected direction, magnitude and timing occur. For suspected failure, stimulation recruits reserve: synthetic ACTH should provoke adrenal cortisol, and a glucagon or insulin-based protocol can assess selected pituitary axes. For suspected excess, suppression tests remove a normal stimulus or add negative feedback: dexamethasone should suppress ACTH-driven cortisol, while oral glucose should suppress growth hormone. The result is conditional on a credible pre-test phenotype; dynamic testing a low-probability patient increases false positives and incidental imaging.

Preparation is part of the assay. Confirm the precise test, fasting and time requirements, recent glucocorticoid or sex-hormone exposure, medicines affecting dexamethasone metabolism, pregnancy, renal or hepatic disease and recent acute illness. Document which cortisol assay and decision limit the laboratory uses. Establish venous access, baseline observations and rescue resources before a test capable of hypoglycaemia, hypotension, vomiting or electrolyte change. Label samples with actual collection times, not planned times, because a misplaced tube can invert interpretation. If the protocol fails, record it as uninterpretable rather than forcing a binary answer.

Interpret the curve with controls and consequences. In a Synacthen test, verify baseline and post-stimulation cortisol against the local modern assay, and remember that recent pituitary injury may retain adrenal responsiveness. After dexamethasone, failure to suppress can reflect non-adherence, absorption or CYP3A interaction as well as endogenous hypercortisolism; repeating with a complementary test may be appropriate. A water-deprivation study requires confirmation of true polyuria and hypotonic urine beforehand, serial weight, sodium and osmolality, and a defined stop rule. Dynamic testing usually establishes presence or absence of a syndrome; ACTH, imaging, genetics or selective venous sampling then localises cause. The person who orders the test must own adverse-event review and the next decision.

Key points

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.

02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.

Suspected adrenal insufficiency

Low or indeterminate morning cortisol with compatible weight loss, postural symptoms, glucocorticoid exposure or pituitary disease may justify stimulation testing after immediate crisis risk is excluded.

Possible cortisol excess

Progressive proximal weakness, easy bruising, wide purple striae, osteoporosis, diabetes or adrenal incidentaloma can justify validated cortisol suppression or free-cortisol testing after exogenous exposure review.

Possible acromegaly

Changing ring or shoe size, coarse facial features, sweating, sleep apnoea and a raised age-adjusted IGF-1 support oral-glucose growth-hormone suppression testing before pituitary localisation.

Hypotonic polyuria

Measured high urine volume with low urine osmolality after excluding glucose, hypercalcaemia, hypokalaemia and diuretics may justify specialist vasopressin-axis testing.

Documented hypoglycaemiaRed flag

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.

Unsafe test candidateRed flag

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.

03Method and interpretationA systematic approach to the test and its findings.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Short Synacthen stimulation testFirst step
Why
Assess adrenal cortisol secretory reserve when basal morning assessment remains indeterminate or specialist confirmation is needed.
Interpretation and limitations
For the licensed adult 30-minute protocol, collect cortisol immediately before and exactly 30 minutes after tetracosactide 250 micrograms IM or IV. Interpret against the current local assay threshold; an apparently adequate response can still be misleading soon after new central ACTH loss.
02
Overnight dexamethasone suppression test
Why
Screen for loss of physiological cortisol suppression in suspected Cushing syndrome or selected adrenal incidentaloma assessment.
Interpretation and limitations
A common UK protocol is dexamethasone 1 mg orally at 23:00 followed by serum cortisol at 09:00. Interpret with the laboratory's validated decision limit; review adherence, absorption, oestrogen, alcohol, depression, acute illness, sleep and CYP3A interactions before labelling non-suppression.
03
Oral glucose growth-hormone suppression test
Why
Confirm autonomous growth-hormone secretion after a persistent raised IGF-1 and compatible phenotype.
Interpretation and limitations
Failure of growth hormone to suppress using the assay-specific criterion supports acromegaly. Diabetes, physiological context and sample timing affect interpretation; MRI follows biochemical confirmation.
04
Water-deprivation and desmopressin test
Why
Differentiate impaired vasopressin secretion, renal resistance and primary polydipsia in confirmed hypotonic polyuria.
Interpretation and limitations
Serial urine concentration, plasma sodium or osmolality and response to desmopressin are interpreted together. Partial disorders overlap and modern copeptin pathways may be preferred locally.
05
Supervised prolonged fast
Why
Capture plasma glucose, insulin, C-peptide, ketones and drug screen during suspected fasting hypoglycaemia.
Interpretation and limitations
Inappropriately detectable insulin and C-peptide during true hypoglycaemia supports endogenous insulin secretion; exogenous insulin and sulfonylurea exposure produce different paired patterns.
06
Mixed-meal test
Why
Reproduce postprandial symptoms and assess biochemical hypoglycaemia after meals, including selected post-bariatric presentations.
Interpretation and limitations
Symptoms require a concurrently low laboratory glucose for interpretation. Insulin values without hypoglycaemia do not diagnose hyperinsulinaemic disease and can lead to unnecessary pancreatic imaging.
07
Insulin tolerance or glucagon stimulation test
Why
Assess selected growth-hormone and ACTH reserve when specialist pituitary evaluation requires a validated provocative test.
Interpretation and limitations
The insulin test requires adequate hypoglycaemia to be interpretable and carries cardiac and seizure risk; centres may use safer alternatives based on contraindications and local validation.

04Clinical next stepsHow the result changes management or prompts escalation.

01Test selectionTurn uncertainty into one questionFirst stepBasal tests and phenotype leave a clinically important endocrine diagnosis unresolved.

1Define whether the suspected abnormality is deficient reserve or autonomous excess, and quantify pre-test probability using phenotype, paired basal hormones and medicines.
2Choose the validated stimulation or suppression test whose result will change treatment, referral or imaging, confirming that a simpler repeat sample cannot answer it.
3Review contraindications, pregnancy, acute illness, organ function and all interfering prescriptions or supplements with endocrinology and biochemistry.
4Book the correct supervised setting, consent process, timed sampling and rescue plan, and name the clinician who will interpret and act on the complete test.

02Test dayProtect protocol integrityThe patient attends for a dynamic endocrine test.

1Reconfirm identity, clinical stability, preparation, fasting, medicine holds and that the actual test matches the request before administering any stimulus or suppressor.
2Record baseline observations, obtain correctly labelled time-zero samples and use a reliable clock for every subsequent specimen.
3Monitor the adverse effects anticipated by that protocol, including glucose, blood pressure, sodium, symptoms and weight where relevant, applying predefined stop rules.
4Document missed, late, haemolysed or insufficient samples and any rescue treatment so the interpreter can declare the study complete, limited or invalid.

03After the curveInterpret before imagingAll dynamic samples and observations are available.

1Apply the laboratory's current assay-specific decision limits and assess the direction and timing of response rather than copying a legacy threshold.
2Review control conditions such as achieved hypoglycaemia, dexamethasone exposure or dehydration endpoint and explain any reason for false-positive or false-negative physiology.
3ConfirmatoryIf abnormal, use paired ACTH or other controller hormones to localise the axis and select confirmatory testing before requesting anatomy.
4Communicate result, uncertainty, interim safety advice and the next endocrine appointment or treatment, ensuring no abnormal study remains an unactioned laboratory report.

05Procedure and medicine safetyRelevant preparation, treatment and contraindications.

Acts as synthetic ACTH to test whether the adrenal cortex can generate an adequate cortisol response.

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.

Provides potent negative feedback to test whether cortisol secretion remains suppressible in suspected endogenous excess.

Dexamethasone for suppression testing

For 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.

Tests renal concentrating response to vasopressin after supervised dehydration has established the endogenous response pattern.

Desmopressin during specialist testing

Give only the locally specified diagnostic dose after the deprivation phase and required samples, with continued sodium, fluid and urine monitoring.

Water retention and hyponatraemia can follow if unrestricted fluid is taken. Do not use an unsupervised therapeutic trial to replace confirmation of true hypotonic polyuria.

06Risks, monitoring and follow-upComplications, safety checks and further assessment.

Maintain a dynamic-test record containing preparation, medicine holds, actual sample times, observations, assay and decision threshold for reproducible interpretation.
During water deprivation, follow weight, thirst, urine output, urine osmolality and plasma sodium or osmolality against explicit stop criteria.
During hypoglycaemic provocation, monitor bedside glucose and neurological state continuously enough to deliver rescue promptly while preserving critical samples.
After Synacthen or dexamethasone testing, record delayed reactions or confounders and communicate temporary steroid safety advice where adrenal insufficiency remains possible.
Audit every completed test to a documented endocrine conclusion, imaging decision and patient notification rather than counting procedure completion alone.

07Special situationsVariants, exceptions and circumstances that change the usual approach.

Direction follows physiology

Stimulation interrogates reserve; suppression interrogates autonomy. Remembering that distinction makes unfamiliar dynamic tests easier to reason through than memorising isolated cut-offs.

A failed protocol is data

If adequate hypoglycaemia was not achieved or dexamethasone was not absorbed, the test did not answer its question. Calling it normal or abnormal creates false certainty.

Modern assays move thresholds

More specific cortisol and growth-hormone assays often read differently from older platforms. A historical textbook number may misclassify a correctly performed modern test.

Recent pituitary injury is special

Adrenals can still respond to synthetic ACTH shortly after ACTH loss. Time since surgery or apoplexy therefore changes what a normal Synacthen response excludes.

Biochemistry precedes geography

Dynamic confirmation identifies a functional syndrome. Imaging before that step finds common incidental lesions and can direct attention to the wrong gland.

08Common pitfallsFrequent interpretation and management errors.

01
Performing a suppression test in a low-probability patient after no medicine or illness review.
02
Using one universal cortisol threshold without checking the laboratory assay and local protocol.
03
Calling an insulin tolerance test normal when adequate hypoglycaemia was never achieved.
04
Starting water deprivation before documenting true polyuria and excluding glucose or electrolyte causes.
05
Imaging the pituitary for a random growth-hormone result instead of confirming raised IGF-1 and failed suppression.
06
Allowing dynamic testing to delay immediate hydrocortisone in suspected adrenal crisis.