A case of primary aldosteronism was unmasked in a 71-year-old man with resistant hypertension when a minimal increase in hydrochlorothiazide dosage precipitated severe hypokalaemia, highlighting the importance of recognizing disproportionate hypokalaemic responses to thiazide treatment as a potential indicator of underlying primary aldosteronism.
Key Findings
Background
A minimal increase in hydrochlorothiazide dosage precipitated severe hypokalaemia in a patient with previously undiagnosed primary aldosteronism.
The patient was a 71-year-old man with resistant hypertension.
Hydrochlorothiazide dose was increased from 12.5 mg to 25 mg — a doubling of dosage described as 'minimal.'
Serum potassium dropped to 1.6 mmol/L, classified as severe hypokalaemia.
The patient had low-normal serum potassium prior to the dose increase, which was a pre-existing clinical signal.
Results
The severe hypokalaemia was accompanied by metabolic alkalosis and rhabdomyolysis.
The thiazide-induced hypokalaemia (1.6 mmol/L) was associated with metabolic alkalosis.
Rhabdomyolysis also developed as a complication of the severe hypokalaemia.
An office electrocardiogram showed hypokalaemia-related changes, prompting urgent referral to the emergency department.
Results
Further evaluation confirmed the diagnosis of primary aldosteronism with an elevated aldosterone-to-renin ratio and a left adrenal nodule.
The aldosterone-to-renin ratio was elevated, a standard biochemical marker for primary aldosteronism screening.
Imaging identified a left adrenal nodule consistent with an aldosterone-producing adenoma.
Primary aldosteronism is described as 'the most common cause of secondary hypertension' but 'remains frequently underdiagnosed in primary care.'
Discussion
A disproportionate hypokalaemic response to thiazide treatment was identified as a potential clinical indicator of underlying primary aldosteronism.
The case illustrates that even a small increase in thiazide dosage can precipitate severe hypokalaemia when primary aldosteronism is present.
Pre-existing low-normal potassium in the setting of resistant hypertension was a warning sign that preceded the acute event.
The authors underscore the role of family physicians in early detection through appropriate screening.
What This Means
This research describes a case report of a 71-year-old man with difficult-to-control high blood pressure who was found to have a condition called primary aldosteronism — a hormonal disorder where the adrenal glands produce too much of a hormone called aldosterone, which causes sodium retention and potassium loss. The diagnosis was uncovered when his doctor made a seemingly small adjustment to his water pill (hydrochlorothiazide), doubling the dose from 12.5 mg to 25 mg. This triggered a dramatic drop in his blood potassium to a dangerously low level (1.6 mmol/L), along with muscle breakdown (rhabdomyolysis) and changes on his heart tracing (ECG), requiring emergency care. Follow-up testing showed elevated levels of the aldosterone hormone relative to the hormone renin, and imaging revealed a growth on his left adrenal gland, both pointing to the diagnosis of primary aldosteronism.
This research suggests that primary aldosteronism is commonly missed in primary care settings, even though it is considered the most frequent cause of secondary (treatable) high blood pressure. A key practical signal highlighted in this case is that patients with resistant hypertension whose potassium is already on the low side of normal may have undiagnosed primary aldosteronism — and that even a modest increase in thiazide diuretic dosage can trigger a severe drop in potassium in such patients. Recognizing this pattern could help doctors identify the condition earlier.
The authors emphasize that family physicians and primary care providers play an important role in catching primary aldosteronism by being alert to warning signs like resistant hypertension combined with low or borderline potassium levels. Earlier diagnosis would allow for more targeted treatment — such as surgery to remove an adrenal tumor or medications that specifically block aldosterone — potentially leading to better blood pressure control and prevention of serious complications.
Del Rey Fernández A, Garcia-Luengo Pensado A, Reyzabal Ereño E. (2026). [Primary aldosteronism in resistant hypertension unmasked by a thiazide-induced severe hypokalaemia].. Anales del sistema sanitario de Navarra. https://doi.org/10.23938/ASSN.1174