suppression test

A suppression test is the mirror image of a stimulation test: instead of asking a gland to turn up, it asks whether the gland can be told to turn down. The clinician applies a signal that should normally switch hormone production off through negative feedback, then checks whether secretion actually falls. If it stubbornly refuses to drop, the gland is escaping normal control.

These tests are the tool of choice when hormone excess is suspected. A high resting level on its own can have benign explanations — stress, time of day, illness — but a gland that cannot be suppressed is behaving autonomously, which is the hallmark of an overactive tumor or a broken feedback loop. The logic is: a normal gland obeys the brake; a diseased one ignores it.

The suppressing signal is usually a hormone or a drug that mimics the body's own feedback messenger, or a load of the very substance the hormone regulates. Failure to suppress redirects the investigation toward locating the rogue source, often combining the result with imaging and with measurement of the controlling pituitary hormone.