Endocrine dynamic function tests - salt suppression tests

Role

Salt/saline suppression testing is a confirmatory test for suspected primary aldosteronism - assessing whether aldosterone secretion appropriately suppresses in response to volume/sodium loading, following an abnormal screening aldosterone-to-renin ratio
Used as the standard confirmatory test once an elevated aldosterone-to-renin ratio has raised suspicion for primary aldosteronism, given the screening test's imperfect specificity requiring confirmation before committing to subtype localisation and treatment

In health, volume expansion (via intravenous saline or high oral sodium loading) suppresses renin and, consequently, aldosterone secretion through normal physiological feedback - in primary aldosteronism, autonomous aldosterone production from an adrenal source (adenoma or bilateral hyperplasia) fails to suppress appropriately despite volume expansion, confirming aldosterone secretion has become independent of the normal renin-angiotensin regulatory axis

Intravenous saline infusion (or oral sodium loading, an alternative protocol) is followed by aldosterone measurement - failure of aldosterone to fall below a defined threshold confirms autonomous secretion
Interfering antihypertensive medications (particularly mineralocorticoid receptor antagonists, and to varying degrees other antihypertensive classes) should be withheld or substituted with agents having less effect on the renin-aldosterone axis before testing, to avoid a false-negative result

Confirmed primary aldosteronism -> proceed to subtype localisation (adrenal CT, and adrenal vein sampling to distinguish unilateral adenoma from bilateral hyperplasia, since this determines whether surgery or medical treatment is appropriate)
Unilateral disease (typically an aldosterone-producing adenoma) is potentially curable with adrenalectomy, while bilateral hyperplasia is managed medically (mineralocorticoid receptor antagonist) - the localisation step is therefore essential to correct treatment selection

Primary aldosteronism as a cause of resistant/secondary hypertension, medication interference with testing (mineralocorticoid receptor antagonists in particular), adrenal vein sampling for subtype localisation, unilateral (surgical) vs bilateral (medical) disease distinction

Confirming primary aldosteronism with salt suppression testing is only the first step - subtype localisation (distinguishing a unilateral aldosterone-producing adenoma from bilateral adrenal hyperplasia, typically via adrenal vein sampling) is essential before deciding between surgical and medical management, since the two subtypes require fundamentally different treatment approaches

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