Secondary Hypertension: When and How to Investigate

Definition and pathophysiology

Secondary hypertension is elevated blood pressure attributable to a specific, identifiable underlying cause. It should be considered in every patient with newly diagnosed or referred hypertension, with particular vigilance when the clinical history includes hypokalaemia.

Endocrine disorders account for many secondary forms. Primary hyperaldosteronism is identified as the most frequent cause, comprising approximately 20–25% of secondary hypertension cases. Coarctation of the aorta and renovascular disease are important non-endocrine causes, particularly in patients with resistant hypertension. Dietary sodium and potassium abnormalities and commonly used over-the-counter medicines may also contribute to apparent treatment resistance.

The diagnosis is clinically important because secondary hypertension may be curable when its cause is clearly established and removed. Cure is not universal, however. Delayed recognition may permit vascular remodelling and renal impairment to develop, leaving persistent hypertension despite correction of the initiating disorder. Earlier diagnosis is associated with a greater likelihood of cure.

When to investigate

General indications

Patients with suggestive signs, symptoms, or medical histories should undergo appropriate screening for secondary hypertension. The initial clinical assessment should establish:

• Whether hypertension is genuinely present

• Factors that may be contributing to elevated blood pressure

• Other cardiovascular risk factors and relevant comorbidities

• Evidence of hypertension-mediated organ damage

• Existing cardiac, cerebrovascular, or renal disease

• The possibility of a secondary cause

The medical and medication histories should specifically assess drugs or substances that may increase blood pressure. The source material does not provide an exhaustive list of such agents.

Resistant hypertension

Resistant hypertension is not regarded as a discrete disease; rather, it is an indicator of increased cardiovascular risk and a setting in which secondary hypertension is relatively common. Patients presumed to have resistant hypertension should undergo a structured assessment and be referred to a specialized centre because the work-up may require expertise and technologies unavailable in primary care.

Before investigating secondary causes, pseudo-resistance must be excluded. This includes:

• Poor adherence to antihypertensive treatment

• White-coat hypertension

• Other contributors to apparently uncontrolled blood pressure

Adherence should initially be explored carefully through direct questioning. Where resources permit, objective assessment may include directly observed treatment or detection of prescribed medications in blood or urine samples.

In patients with an estimated glomerular filtration rate below 30 mL/min/1.73 m2, an appropriately up-titrated loop diuretic is necessary before resistant hypertension can be defined. The guideline framework does not use the terms “controlled resistant hypertension” for blood pressure at target on four or more drugs or “refractory hypertension” for blood pressure above target despite five or more drugs.

Screening for primary aldosteronism

Screening for primary aldosteronism with renin and aldosterone measurements should be considered in all adults with confirmed hypertension, defined in the cited recommendation as blood pressure ≥140/90 mmHg. Screening is particularly pertinent in the presence of hypokalaemia or treatment resistance.

Hypokalaemia should not be permitted when assessing dietary electrolyte intake, because it can increase blood pressure and may affect interpretation of the clinical picture. A 24-hour urine collection for electrolytes can be used to quantify sodium and potassium intake, preferably while the patient is receiving a stable diuretic dose.

Other clinical contexts

Secondary causes should be pursued when abnormalities are identified on initial testing or when particular symptoms or physical findings suggest a specific diagnosis. Patients who remain resistant to treatment during follow-up have higher rates of secondary causes, especially:

• Obstructive sleep apnoea

• Hyperaldosteronism

• Renovascular disease

Rarer causes should be assessed in specialized hypertension centres.

Clinical evaluation and physical examination

The purpose of the clinical evaluation is broader than identifying a cause of high blood pressure. It should also determine cardiovascular risk, comorbid disease, target-organ damage, and whether there is established cardiac, cerebrovascular, or renal disease.

Physical examination findings and symptoms that suggest a secondary cause should prompt targeted investigations. The supplied material does not provide a detailed symptom-by-symptom physical examination protocol or the specific physical signs associated with each secondary hypertensive disorder.

Blood pressure assessment requires attention to the possibility of white-coat hypertension and, where relevant, confirmation outside the clinic. In patients with diabetes, blood pressure should be measured regularly under standardized conditions and confirmed in both arms using multiple readings obtained on separate days. In patients with cardiovascular disease and blood pressure >180/110 mmHg, diagnosis at a single visit may be reasonable.

Initial diagnostic strategy

A practical investigation pathway consists of four stages:

• Confirm that hypertension is persistent and not a measurement-related phenomenon.

• Identify pseudo-resistance, particularly non-adherence and white-coat hypertension.

• Perform routine laboratory and cardiovascular assessment for target-organ damage, comorbidities, and clues to secondary disease.

• Undertake focused testing according to the clinical presentation, abnormal baseline results, or treatment resistance.

Routine laboratory investigations

The minimum baseline assessment is intended to detect target-organ damage, identify comorbidities, estimate cardiovascular risk, assist drug selection, and provide clues to secondary causes.

The basic laboratory set is also described as including renal function, electrolytes, calcium, glucose, haemoglobin, lipid profile, urinalysis, and ECG. The more detailed minimum panel additionally specifies thyroid-stimulating hormone and urinary albumin-to-creatinine ratio.

Abnormal initial results should lead to further investigation. If local resources do not permit all minimally recommended tests, this should not prevent blood-pressure reduction.

Focused investigations for secondary causes

Primary hyperaldosteronism

The recommended screening approach is measurement of renin and aldosterone, with interpretation in the context of potassium status and medications or conditions that influence the aldosterone-to-renin ratio. The supplied material does not provide specific threshold values, medication-withdrawal protocols, confirmatory testing procedures, adrenal imaging criteria, or subtype-directed treatment.

Renovascular hypertension

Renovascular disease is an important non-endocrine cause of resistant hypertension and should be considered when the clinical history, examination, or baseline renal assessment is suggestive. The source material identifies renal function testing, urinalysis, and urinary albumin-to-creatinine ratio as part of the initial assessment and indicates that further testing should be guided by abnormal findings or clinical suspicion. It does not specify a preferred imaging modality or diagnostic threshold.

Obstructive sleep apnoea

Obstructive sleep apnoea is particularly relevant in patients whose hypertension remains resistant to treatment. The material recommends a more dedicated search for this condition in such patients but does not describe symptom criteria, sleep studies, or treatment.

Phaeochromocytoma and paraganglioma

These are included among the causes for which targeted evaluation may be required when clinical features suggest secondary hypertension. In pregnancy-related hypertensive assessment, plasma and urinary fractionated metanephrine assays and adrenal ultrasound may be considered. The source does not provide a general diagnostic algorithm, biochemical thresholds, or management details for these tumours.

Other endocrine and structural causes

The routine electrolyte and endocrine profile may provide clues to disorders such as:

• Cushing’s disease

• Polycythaemia-associated hypertension

• Hyperparathyroidism

• Hyperthyroidism

Coarctation of the aorta is identified as another common non-endocrine contributor to resistant hypertension. The supplied material does not define the specific examination findings or imaging tests used to establish these diagnoses.

ECG and cardiovascular assessment

A 12-lead ECG is part of the routine initial work-up. It may identify hypertension-mediated cardiac damage, including:

• Left ventricular hypertrophy

• Left atrial enlargement

It may also reveal an irregular pulse or associated disease such as atrial fibrillation or prior acute myocardial infarction.

The ECG therefore has both a target-organ role and a role in identifying comorbid cardiovascular disease that influences overall risk assessment and treatment planning.

Imaging and other complementary tests

Further tests should be selected according to the initial findings and clinical suspicion rather than performed indiscriminately. Imaging and complementary investigations are directed toward:

• Established target-organ damage

• Comorbid conditions

• Possible secondary causes

• Existing cardiac, cerebrovascular, or renal disease

In patients with suspected pulmonary hypertension associated with left heart disease, the diagnostic approach may include echocardiography, ECG, and BNP or NT-proBNP measurement. If pulmonary hypertension is mild and left heart disease is predominant, additional testing may not be necessary. Otherwise, chronic thromboembolic pulmonary hypertension and significant lung disease should be excluded with a ventilation–perfusion scan and pulmonary function tests; cardiac magnetic resonance imaging may be considered in selected cases.

A transthoracic echocardiogram is recommended when patients develop new exertional dyspnoea, fatigue, or angina and pulmonary hypertension is suspected. Echocardiography alone cannot confirm pulmonary hypertension. Patients with a high echocardiographic probability require right-heart catheterization for confirmation and should be referred to a specialist pulmonary hypertension service.

Laboratory assessment in pregnancy

For hypertensive disorders of pregnancy, recommended basic tests include:

• Urinalysis

• Complete blood count

• Haematocrit

• Liver enzymes

• Serum creatinine

• Serum uric acid

Proteinuria should be assessed early in pregnancy to detect pre-existing renal disease and again in the second half of pregnancy to screen for pre-eclampsia.

Additional investigations may include adrenal ultrasound, plasma and urinary fractionated metanephrines, and Doppler ultrasound of the uterine arteries after 20 weeks’ gestation. In clinically suspected pre-eclampsia, an sFlt-1-to-PlGF ratio <38 can reliably exclude development of pre-eclampsia over the subsequent 7 days. In women with chronic hypertension, PlGF testing may assist in ruling out pre-eclampsia between 20 and 36 weeks.

Treatment and management

Treat the blood pressure while investigating the cause

Investigation for secondary hypertension should not delay blood-pressure reduction when the diagnosis cannot immediately be established or when all recommended laboratory tests are not available. Management should address both the elevated blood pressure and the underlying cause once identified.

The source material does not provide specific antihypertensive regimens, drug doses, treatment targets, or procedural interventions for individual secondary causes.

Correct reversible contributors

Management should include identification and correction of factors contributing to pseudo-resistance or true resistance. This includes:

• Improving adherence to prescribed treatment

• Excluding white-coat hypertension

• Reviewing drugs and substances that may elevate blood pressure

• Addressing dietary sodium and potassium issues

• Ensuring adequate diuretic therapy in patients with substantially reduced eGFR

In patients with suspected resistant hypertension, referral to a specialized centre is recommended.

Cause-directed management

Secondary hypertension is, by definition, potentially remediable. When the underlying cause is unequivocally identified and removed, hypertension may resolve; however, persistent elevation can remain because of vascular remodelling or renal impairment arising during delayed diagnosis.

The provided material does not specify the operative, interventional, endocrine, respiratory, or pharmacological treatments for individual secondary causes.

Guideline recommendations

The cited recommendations can be summarized as follows:

Prognosis and follow-up

The prognosis depends on the underlying cause, the duration of hypertension, and the extent of established vascular, renal, or cardiac injury. Early diagnosis improves the likelihood of cure. Delayed diagnosis may result in vascular remodelling and renal dysfunction, with residual hypertension even after the initiating cause has been removed.

Patients with hypertensive emergencies remain at high risk despite improved survival over recent decades and should be screened for secondary hypertension.

Follow-up should reassess:

• Blood-pressure control using appropriate repeated or out-of-office measurements when indicated

• Treatment adherence and persistence

• Renal function and electrolytes

• Urinary abnormalities and albuminuria when present

• Cardiovascular risk factors and comorbidities

• Evidence of ongoing or progressive target-organ damage

• The need for further investigation if blood pressure remains resistant

Persistent treatment resistance should prompt renewed evaluation for pseudo-resistance and secondary causes, particularly obstructive sleep apnoea, hyperaldosteronism, and renovascular disease.