Why one number is not enough
Hormone levels swing through the day and respond to stress, food, and sleep. A borderline value caught at the wrong moment can look normal in real disease or abnormal in a healthy person. Dynamic testing solves this by refusing to trust a still picture. Instead it deliberately disturbs an endocrine axis and measures the reply, the way you might tap a wall to hear whether it is solid or hollow.
The whole field rests on one idea: a stimulation test asks a gland suspected of being underactive, “Can you respond when pushed?”, while a suppression test asks a gland suspected of being overactive, “Can you be switched off when told to stop?” A normal axis obeys; a diseased one does not.
Stimulation: can the gland answer?
When you suspect hormone deficiency, you give the natural signal that should wake the gland and watch whether it produces hormone. The classic example is the ACTH stimulation test for Addison's disease: you inject synthetic ACTH, the signal the pituitary normally uses to drive the adrenal cortex, then measure cortisol before and after. Healthy adrenals jump; damaged adrenals stay flat. A flat response confirms the gland itself has failed.
Suppression: can the gland be told to stop?
When you suspect hormone excess, you exploit negative feedback: in a healthy axis, giving extra hormone tells the brain “there's plenty already,” so it shuts production down. The dexamethasone suppression test for Cushing's syndrome gives a small dose of a powerful synthetic glucocorticoid at night and checks cortisol the next morning. A normal axis reads the dexamethasone as cortisol and switches off; in Cushing's the cortisol stays high because a tumor is making it without listening to feedback.
STIMULATION (suspect deficiency) SUPPRESSION (suspect excess)
give the driving signal give extra hormone
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measure gland output measure gland output
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rises -> gland is healthy falls -> axis obeys feedback (normal)
flat -> gland has failed stays high -> autonomous excess (disease)
Example axis (HPA):
hypothalamus --CRH--> pituitary --ACTH--> adrenal --CORTISOL--> body
^------------------ negative feedback ------------------|