Measuring hormones: stimulation and suppression tests !

Measurement
Exam priority: medium

If a hormone is thought to be missing, the gland is stimulated; if it is thought to be in excess, the test tries to suppress it.

Updated 2026-11-01

Also called: Dynamic tests, Immunoassays

See it in 3D: Right adrenal gland · Pituitary gland

What it is

A single hormone level often misleads because many hormones are pulsatile, circadian or change with stress. So the pair (trophic hormone and final hormone) is always measured and, when needed, the axis is challenged. Dynamic tests use feedback: a healthy gland responds to stimulation and switches off with suppression; a diseased gland does not.

Key facts

Immunoassays (ELISA, chemiluminescence) use antibodies; they can fail through interference (biotin, heterophile antibodies).
Hook effect: with extremely high prolactin (macroprolactinoma) a two-site assay can read falsely low; the sample is re-run diluted.
Estrogen raises CBG and TBG: high total cortisol and total T4 without excess free hormone.
Timing matters: cortisol and ACTH at 8 a.m.; testosterone in the morning; a single GH value means little because of its pulsatile secretion.

Hormone axes and feedback

Lesion Addison diseasePattern CRH ↑ · ADH ↑ · ACTH ↑ · Cortisol ↓

Primary hypofunction: Addison disease

Pattern: CRH ↑ · ADH ↑ · ACTH ↑ · Cortisol ↓

The target gland fails: its hormone falls and, without negative feedback, the pituitary and hypothalamic hormones rise. Cortisol ↓ with ACTH ↑ → hyperpigmentation (MSH). Aldosterone also falls: hyperkalemia, hyponatremia, hypotension and salt craving.

Exam rule: if the pituitary hormone moves opposite to the gland hormone, the problem is in the gland (primary); if they move together, it is central (pituitary or hypothalamus).

Stimulates (+)
Inhibits / negative feedback (−)
Level ↑
Level ↓

Illustrative schematic drawn by Durgical: simplified axes and classic lab patterns; not for clinical decisions.

Classic dynamic tests

Test	What for	Normal response	Abnormal result
Synthetic ACTH stimulation	Adrenal insufficiency	Cortisol clearly rises	Flat response
Low-dose dexamethasone suppression	Screening for Cushing syndrome	Morning cortisol suppressed	Fails to suppress
High-dose dexamethasone suppression	Source of ACTH-dependent Cushing	—	Suppresses: pituitary; does not: ectopic
Oral glucose load with GH	Acromegaly	GH falls	GH does not fall or rises
Water deprivation	Polyuria with dilute urine	Urine concentrates	Urine stays dilute (diabetes insipidus)
Insulin-induced hypoglycemia	GH and ACTH reserve	GH and cortisol rise	Low response (hypopituitarism)

Clinical notes

Before a suppression test, drugs that change dexamethasone metabolism or CBG (oral contraceptives, enzyme inducers) are reviewed.
In recent secondary adrenal insufficiency the ACTH test can be normal because the gland has not yet atrophied.

Test yourself

Think of the answer, then tap the question to see it.

Why does dexamethasone suppress cortisol in a healthy person but not in Cushing syndrome from an adrenal tumor?

Dexamethasone brakes CRH and ACTH by feedback. The adrenal tumor makes cortisol on its own, without needing ACTH, so it does not switch off.

Why does high-dose dexamethasone usually suppress pituitary Cushing disease but not ectopic ACTH?

The corticotroph adenoma keeps some feedback sensitivity, only with a higher set point; an ectopic ACTH-secreting tumor lacks that regulation.

Why is IGF-1 measured instead of a single GH level when looking for acromegaly?

GH is secreted in pulses and swings widely through the day; IGF-1 has a long half-life and reflects integrated GH secretion.

What is the hook effect and when does it matter?

At an extreme concentration the antigen saturates both assay antibodies and the result reads falsely low; it matters in macroprolactinomas and is avoided by diluting the sample.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

In Biochemistry

← All of physiology

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: Cortisol Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2024); Hypercortisolism (Cushing Syndrome) (StatPearls (NCBI Bookshelf), 2024); Adrenal Insufficiency (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 75). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.