Low Blood Pressure: What Could Be Causing It

What Blood Pressure Reading Counts as "Low"

A systolic reading below 90 mmHg or a diastolic reading below 60 mmHg meets the clinical definition of hypotension, according to the American Heart Association. But the number alone tells only half the story.

Some people walk around with a resting blood pressure of 85/55 mmHg and feel perfectly fine. Elite endurance athletes, younger women, and individuals with naturally low vascular tone often sit below the 90/60 threshold without any pathology. The distinction between "constitutional" low blood pressure and clinically significant hypotension comes down to symptoms: dizziness, lightheadedness, blurred vision, fatigue, nausea, or syncope. A 2019 analysis published in the European Heart Journal found that among 30,369 adults in the Malmö Diet and Cancer Study, those with systolic blood pressure consistently below 100 mmHg had increased all-cause mortality only when accompanied by comorbidities or symptomatic episodes [1]. Without symptoms, low readings alone rarely warrant aggressive workup.

The 2017 ACC/AHA blood pressure guidelines focus almost entirely on hypertension thresholds, which has contributed to underrecognition of clinically meaningful hypotension in primary care [2]. Your provider should measure blood pressure in both arms and in both sitting and standing positions before concluding the reading is benign.

Medications: The Single Most Common Reversible Cause

More than half of symptomatic hypotension cases in adults over 60 trace back to a prescribed medication. That is the finding your doctor should investigate first.

The classes most frequently implicated include loop diuretics (furosemide), thiazides (hydrochlorothiazide), ACE inhibitors, angiotensin receptor blockers, calcium channel blockers (amlodipine), alpha-1 blockers (tamsulosin, doxazosin), and nitrates. A population-based cohort study of 765,000 UK primary care patients showed that initiation of antihypertensive therapy increased falls by 18% in the first 14 days, largely mediated through orthostatic hypotension [3]. Psychotropic medications compound this risk. Tricyclic antidepressants, phenothiazine antipsychotics, and even SSRIs at high doses carry alpha-adrenergic blocking properties that lower peripheral resistance. A systematic review in the Journal of Clinical Psychopharmacology identified orthostatic hypotension in 11.5% of patients on quetiapine and 23.5% on clozapine [4].

Opioids represent another underrecognized contributor. Morphine and fentanyl cause histamine-mediated vasodilation and blunt the baroreceptor reflex. Patients taking multiple blood-pressure-lowering agents simultaneously (a common scenario in older adults with polypharmacy) face compounding effects that may not surface during a single in-office reading. Home blood pressure monitoring over 3 to 5 days, ideally with position changes logged, gives a more accurate picture. Dose reduction or medication substitution resolves many cases entirely.

Orthostatic Hypotension: When Standing Drops Your Pressure

Orthostatic hypotension (OH) is defined as a sustained drop in systolic blood pressure of 20 mmHg or more (or diastolic of 10 mmHg or more) within 3 minutes of standing, per the 2011 consensus statement from the American Autonomic Society and the European Federation of Autonomic Societies [5]. It affects an estimated 5 to 30% of community-dwelling older adults, depending on the population studied, and prevalence rises steeply in those with diabetes, Parkinson disease, and chronic kidney disease [6].

The pathophysiology splits into two buckets. Neurogenic OH results from failure of the autonomic nervous system to compensate for gravitational blood pooling, a hallmark of conditions like pure autonomic failure, multiple system atrophy, and diabetic autonomic neuropathy [7]. Non-neurogenic OH results from volume depletion, medications, or venous pooling in the absence of nerve damage.

Distinguishing between the two matters because treatment diverges sharply. Dr. Horacio Kaufmann, director of the Dysautonomia Center at NYU Langone, has noted: "Neurogenic orthostatic hypotension is a disease of the nerves, not the blood vessels. Treating it with fluids alone is like treating a broken thermostat by opening a window" [7]. Neurogenic cases may require midodrine (an alpha-1 agonist, typically 2.5 to 10 mg three times daily) or droxidopa (100 to 600 mg three times daily), both of which are FDA-approved for symptomatic neurogenic OH [8]. Non-neurogenic cases often improve with increased sodium intake (6 to 10 g per day if renal and cardiac function allow), compression garments, and slow positional transitions.

Dehydration and Volume Depletion

This cause is both the simplest and the most underestimated. Blood pressure depends on circulating blood volume. Reduce that volume, and pressure falls.

Dehydration from inadequate fluid intake, prolonged heat exposure, vomiting, diarrhea, or excessive sweating reduces preload to the heart. A controlled crossover trial published in Physiology Reports demonstrated that a 1.5% reduction in body mass through fluid restriction dropped mean systolic pressure by 7 mmHg and impaired cerebral autoregulation in healthy young adults [9]. In older adults with blunted thirst response, the effect is magnified.

Hemorrhage (acute blood loss from trauma, gastrointestinal bleeding, or surgical complications) causes rapid intravascular volume depletion. A hemoglobin drop of 2 g/dL or more should trigger evaluation for occult bleeding. Third-spacing, where fluid shifts into the interstitial or peritoneal space (common in cirrhosis, nephrotic syndrome, and severe burns), can cause functional hypovolemia even when total body water is normal or elevated. The American College of Gastroenterology guidelines recommend a systolic blood pressure target above 90 mmHg as a triage threshold in upper GI bleeding, with crystalloid resuscitation initiated before endoscopy [10].

GLP-1 receptor agonists, now widely prescribed for obesity and type 2 diabetes, can contribute indirectly. Nausea and reduced oral intake on semaglutide or tirzepatide may lead to volume contraction, especially during dose titration. This is not hypotension caused by the drug itself but rather a downstream consequence of reduced caloric and fluid intake.

Endocrine Causes: Adrenal, Thyroid, and Beyond

Hormone deficiencies produce some of the most overlooked causes of chronic low blood pressure. The pattern often goes unrecognized for months or years.

Primary adrenal insufficiency (Addison disease) results in deficient cortisol and aldosterone production. Aldosterone is the primary mineralocorticoid controlling sodium reabsorption and potassium excretion, so its absence leads to sodium wasting, hyperkalemia, and contracted plasma volume. Blood pressure drops follow. The Endocrine Society 2016 clinical practice guideline estimates the prevalence of primary adrenal insufficiency at 100 to 140 per million in developed countries [11]. Diagnosis requires an early-morning cortisol level (drawn before 9 AM) followed by a cosyntropin stimulation test if the cortisol is below 500 nmol/L. Hyperpigmentation, salt craving, and chronic fatigue are classic but not always present.

Hypothyroidism, particularly when severe, reduces cardiac output and heart rate, contributing to low blood pressure. Pheochromocytoma, though classically associated with hypertension, can produce episodic hypotension between catecholamine surges. Hypopituitarism causes secondary adrenal and thyroid insufficiency simultaneously, producing a compound hemodynamic effect. For patients with new-onset unexplained hypotension, a basic endocrine panel (AM cortisol, TSH, free T4) is a low-cost, high-yield screening step.

Cardiac Causes: Pump Failure, Valvular Disease, and Arrhythmias

When the heart cannot generate adequate forward flow, blood pressure drops regardless of volume status. This category carries the highest acuity risk.

Severe aortic stenosis restricts left ventricular outflow, producing fixed low cardiac output. Heart failure with reduced ejection fraction (HFrEF), defined as an EF below 40%, directly impairs the pump mechanism [12]. Bradyarrhythmias (sick sinus syndrome, high-grade AV block) reduce cardiac output by limiting heart rate below the compensatory threshold. Tachyarrhythmias, particularly atrial fibrillation with rapid ventricular response, reduce diastolic filling time and paradoxically lower effective output despite the fast rate. Cardiac tamponade and massive pulmonary embolism cause acute obstructive hypotension requiring emergent intervention.

The 2022 AHA/ACC/HFSA guideline for heart failure management states: "In patients with HFrEF and symptomatic hypotension, the dose of GDMT [guideline-directed medical therapy] should be reduced in a stepwise fashion, prioritizing agents with the strongest mortality benefit (ARNI, beta-blocker, MRA) for retention" [12]. In practice, this means a patient on carvedilol, sacubitril/valsartan, and spironolactone who develops symptomatic hypotension will typically have spironolactone reduced first, followed by the ARNI.

An echocardiogram is the single most informative test when cardiac causes are suspected. It quantifies ejection fraction, identifies valvular pathology, and detects pericardial effusion in a single study.

Sepsis and Infection

Infection-driven hypotension is a medical emergency. Sepsis triggers a systemic inflammatory cascade that causes vasodilation, capillary leak, and myocardial depression simultaneously.

The Surviving Sepsis Campaign 2021 guidelines define sepsis-induced hypotension as a mean arterial pressure (MAP) below 65 mmHg or a systolic below 90 mmHg after adequate fluid resuscitation, or a lactate above 2 mmol/L [13]. Septic shock, the most severe form, carries an in-hospital mortality of 25 to 30% in recent multicenter data. The ProCESS trial (N=1,341) showed no mortality difference between protocolized early goal-directed therapy and usual care, but both arms achieved MAP targets above 65 mmHg within 6 hours, confirming that timely resuscitation itself is the critical variable [14].

Any acute infection in an older, immunocompromised, or chronically ill patient who presents with new hypotension warrants blood cultures, a lactate level, and broad-spectrum antibiotics within the first hour. Urinary tract infections, pneumonia, and intra-abdominal infections are the three most common sources in the community setting.

Autonomic Neuropathy and Neurological Conditions

Damage to the autonomic nerves that regulate vascular tone produces a distinct pattern of blood pressure instability. This is different from simple dehydration or medication effects.

Diabetic autonomic neuropathy is the most prevalent form. A cross-sectional study of 2,230 patients with type 2 diabetes found cardiovascular autonomic neuropathy in 34.3% of participants when tested with standardized Ewing battery methods [15]. The risk increases with diabetes duration and poor glycemic control (HbA1c above 8%). Parkinson disease produces autonomic failure in 30 to 58% of patients, often predating motor symptoms by years [7]. Multiple system atrophy causes the most severe form of neurogenic orthostatic hypotension, with systolic drops exceeding 50 mmHg common.

Spinal cord injuries above the T6 level disrupt sympathetic outflow, producing both resting hypotension and episodic autonomic dysreflexia. Amyloidosis (both AL and transthyretin forms) infiltrates autonomic ganglia and peripheral nerves, causing progressive orthostatic hypotension alongside peripheral neuropathy and sometimes cardiac infiltration. Evaluation includes heart rate variability testing, tilt table testing, and in selected cases, sudomotor function assessment and skin biopsy for small fiber neuropathy [7].

Other Causes: Nutritional Deficiencies, Allergic Reactions, and Pregnancy

Several additional mechanisms deserve mention because they are frequently missed during initial evaluation.

Vitamin B12 deficiency causes megaloblastic anemia, reducing oxygen-carrying capacity and cardiac output. Severe iron deficiency anemia produces the same hemodynamic effect through a different pathway. Anaphylaxis causes acute distributive hypotension through massive histamine and tryptase release, typically within minutes of allergen exposure. Epinephrine 0.3 mg intramuscular (via auto-injector) is first-line treatment, per the 2020 World Allergy Organization anaphylaxis guidance document [16].

Pregnancy produces physiological hypotension during the second trimester as progesterone-mediated vasodilation outpaces the rise in blood volume. Systolic pressure typically drops 5 to 10 mmHg between weeks 12 and 24 before returning to baseline in the third trimester [17]. This is normal. New-onset hypotension in the third trimester, particularly when accompanied by vaginal bleeding or abdominal pain, warrants immediate evaluation for placental abruption.

Prolonged bed rest (common during hospitalization) deconditions the baroreceptor reflex within 3 to 5 days, producing orthostatic hypotension at mobilization. Early ambulation protocols in hospital medicine guidelines specifically target this mechanism [18].

How Doctors Diagnose the Cause of Low Blood Pressure

The diagnostic workup follows a structured approach: history first, then targeted testing based on the clinical pattern.

Step one is a complete medication reconciliation, including over-the-counter supplements, herbal products, and recreational substances. Step two is orthostatic vital signs: blood pressure and heart rate measured supine, then at 1 minute and 3 minutes after standing. A systolic drop of 20 mmHg or more with an appropriate heart rate rise (above 15 bpm) suggests non-neurogenic OH. A drop without heart rate compensation points toward neurogenic causes [5].

Baseline labs include a complete blood count (to detect anemia), basic metabolic panel (sodium, potassium, creatinine, glucose), and thyroid function. If adrenal insufficiency is suspected, an AM cortisol and ACTH level guide the next step. An echocardiogram is indicated when there is a cardiac murmur, history of heart failure, abnormal ECG, or no clear non-cardiac explanation. Tilt table testing, performed in a controlled setting with continuous blood pressure and heart rate monitoring, provides the definitive diagnosis for autonomic causes and vasovagal syncope. Ambulatory 24-hour blood pressure monitoring captures patterns missed by office readings, including nocturnal dipping and postprandial hypotension.

Treatment Depends Entirely on the Cause

There is no single pill for low blood pressure. Treatment targets the specific mechanism producing the hypotension.

For medication-induced cases, dose reduction or substitution resolves the problem. For volume depletion, oral or intravenous fluid replacement restores circulating volume. For neurogenic orthostatic hypotension, the FDA-approved options are midodrine (starting at 2.5 mg three times daily, titrated to 10 mg) and droxidopa (starting at 100 mg three times daily) [8]. Fludrocortisone (0.1 to 0.3 mg daily), a synthetic mineralocorticoid, expands plasma volume and is used off-label for both neurogenic and non-neurogenic OH. Non-pharmacological measures include increasing dietary sodium to 6 to 10 g per day (with physician approval), wearing waist-high compression stockings (30 to 40 mmHg), sleeping with the head of the bed elevated 10 to 15 degrees, and avoiding large carbohydrate-heavy meals that trigger postprandial blood pooling.

For cardiac causes, treatment addresses the underlying structural or rhythm abnormality: valve replacement for critical aortic stenosis, pacemaker for symptomatic bradycardia, or optimization of heart failure pharmacotherapy. For sepsis, the 2021 Surviving Sepsis Campaign recommends 30 mL/kg of crystalloid within the first 3 hours and initiation of norepinephrine as the first-line vasopressor if MAP remains below 65 mmHg after fluid resuscitation [13].