# Dynamic Endocrine Function Tests

> Dynamic endocrine function tests for NEET-PG Medicine covers dexamethasone suppression, ACTH stimulation, the growth hormone glucose load and water deprivation.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/dynamic-endocrine-function-tests
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Williams Textbook of Endocrinology; Harrison's Principles of Internal Medicine
- How to cite: "Dynamic Endocrine Function Tests", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/dynamic-endocrine-function-tests

## Direct answer

Dynamic endocrine function tests challenge a hormone axis with a stimulus or suppressant to show whether its feedback control is intact. Each test has an expected normal response, and a failure to suppress or to rise points to a specific site of disease, such as the pituitary, adrenal gland or kidney. Classic examples include dexamethasone suppression, ACTH stimulation and the glucose load for growth hormone.

## What you must remember

- **Low-dose dexamethasone:** a morning cortisol below about 50 nmol/L after 1 mg overnight indicates normal suppression, so failure to suppress favours Cushing's syndrome.
- **High-dose dexamethasone:** suppresses cortisol in pituitary Cushing's disease but not in ectopic ACTH syndrome or adrenal tumours, which localises the source.
- **ACTH stimulation:** a peak cortisol above about 500 nmol/L after cosyntropin indicates adequate reserve, and no rise with a high basal ACTH signals primary adrenal insufficiency.
- **Glucose load for GH:** growth hormone should fall below 1 ng/mL during an oral 75 g glucose load, and failure to suppress defines acromegaly.
- **Water deprivation:** urine osmolality rising well above plasma osmolality indicates intact concentration, which points to primary polydipsia.
- **Desmopressin response:** a rise in urine osmolality of more than 50 per cent after desmopressin favours central diabetes insipidus, while no rise suggests nephrogenic disease.
- **Saline infusion:** aldosterone normally falls after about 2 litres of saline over 4 hours, and failure to suppress supports primary aldosteronism.

## Choosing the test and reading the pattern

A 45-year-old woman with central obesity, purple striae and hypertension has a raised late-night cortisol. The question is whether the source is pituitary, ectopic or adrenal. Each dynamic test targets one link in the feedback loop, so the choice depends on the step being tested.

| Test | Principle | Normal response | Diagnostic pattern |
|---|---|---|---|
| Low-dose dexamethasone | Suppress cortisol | Cortisol below about 50 nmol/L | No suppression in Cushing's |
| High-dose dexamethasone | Test pituitary suppressibility | Cortisol falls by more than half | Suppresses only in pituitary disease |
| ACTH stimulation | Test adrenal reserve | Peak above about 500 nmol/L | No rise in primary insufficiency |
| Oral glucose load | Test GH suppression | GH below 1 ng/mL | No suppression in acromegaly |
| Water deprivation | Test ADH and renal response | Urine concentrates appropriately | Low urine osmolality, no rise after desmopressin |

A normal response excludes the lesion that the test targets, but a single failed result is never enough on its own. Cut-offs differ between texts and laboratories, so every result must be read against the stated threshold and the clinical picture.

## Where the exam tries to catch you

Most errors come from treating one abnormal number as a diagnosis rather than as a clue to the level of the lesion.

- **Trap:** reading a failed suppression as proof of Cushing's syndrome — **Correct:** obesity, depression and alcohol excess can also fail suppression, so a second test is needed.
- **Trap:** using the high-dose test to diagnose Cushing's — **Correct:** it localises the source once the syndrome is confirmed, and it does not make the diagnosis by itself.
- **Trap:** relying on the short cosyntropin test to exclude secondary insufficiency — **Correct:** early secondary insufficiency can respond normally, so basal ACTH is essential.

## Frequently asked questions

### What cortisol result on the low-dose dexamethasone test suggests Cushing's syndrome?

Normal suppression after 1 mg of dexamethasone overnight gives a morning cortisol below about 50 nmol/L. Failure to suppress below this level suggests Cushing's syndrome, but obesity, depression and alcohol excess can also cause false positives. Confirmation with a second test is therefore needed.

### How does high-dose dexamethasone help localise Cushing's syndrome?

High-dose dexamethasone suppresses cortisol in pituitary Cushing's disease but not in ectopic ACTH syndrome or adrenal tumours. This separation helps localise the source once Cushing's syndrome is confirmed. The test is an aid to localisation rather than a stand-alone diagnosis.

### What does a failed cosyntropin test show?

A failed cosyntropin test, with no adequate cortisol rise after ACTH stimulation, indicates adrenal insufficiency. In primary adrenal insufficiency the basal ACTH is raised, whereas in secondary insufficiency it is low or inappropriately normal. Early secondary insufficiency can still respond normally, so basal ACTH is essential.

### Why is an oral glucose load used to test growth hormone?

An oral glucose load is used because normal growth hormone secretion is suppressed by hyperglycaemia, which allows assessment of autonomous secretion. Growth hormone should fall below 1 ng/mL in healthy people, and failure to suppress defines acromegaly. Levels are measured at timed intervals during the test.

### How does desmopressin distinguish central from nephrogenic diabetes insipidus?

Desmopressin distinguishes the two by testing whether the kidney responds to a synthetic ADH analogue. Central diabetes insipidus shows a rise in urine osmolality of more than 50 per cent after desmopressin, whereas nephrogenic disease shows little or no rise. Cut-offs vary between texts, so the pattern should be read with the full set of results.