The goal of this observational study is to see if there is a cyclical or exaggerated diurnal variation in aldosterone production in people with Primary Aldosteronism.

The main questions it aims to answer are:

• Can we diagnose more people if we used 24 hour urine measurements?
• In those with high amounts of aldosterone in their urine, is there a variable pattern to their aldosterone production?

Participants will have a 24 hour urine measurement. They will also have multiple blood tests throughout the day to study the variability in aldosterone secretion.

Inclusion Criteria:

• People with clinically suspected PA but have not met criteria for diagnosis. Suspicion based on low-renin (renin activity <0.5 nmol/h/L or renin mass <5 ng/L), plasma sodium > 140mmol/L or plasma potassium < 4mmol/L.
• Patients who have been diagnosed with PA and had previous aldosterone samples <277 pmol/L, a level which would normally not qualify for confirmatory testing.
• Patients with aldosterone results done at different times that indicate variability in production.
• Willing to consent and participate in the study.

Exclusion Criteria:

• Inability to withdraw β-adrenoceptor antagonist therapy for 2 weeks.
• People on end of life treatment.

People with hypertension.