POTS, or postural orthostatic tachycardia syndrome, doesn’t have a single cause. It results from a breakdown in the body’s ability to manage blood flow when you stand up, and that breakdown can happen through several different pathways. Some people develop it after a viral infection, others have it linked to an autoimmune process or a connective tissue disorder, and for many, multiple factors overlap. Understanding which mechanism is driving your symptoms can shape how it’s managed.
What Happens in the Body
When you stand, gravity pulls about half a liter of blood into your legs and abdomen. In a healthy body, your nervous system responds within seconds: blood vessels in your lower body tighten, your heart rate increases slightly, and hormones adjust to keep blood flowing to your brain. In POTS, this system fails. Blood pools in the lower body, stroke volume drops, and the heart compensates by racing, often increasing by more than 30 beats per minute within 10 minutes of standing (or exceeding 120 beats per minute). In adolescents, the threshold is even higher: 40 beats per minute.
The tachycardia isn’t the disease itself. It’s a compensatory response to inadequate blood return. Your heart is trying to make up for the fact that not enough blood is getting back up to the chest and brain. That’s why POTS causes not just a fast heartbeat but also lightheadedness, brain fog, fatigue, and sometimes fainting.
Nerve Damage in the Lower Body
One of the most well-characterized forms of POTS involves damage to the small nerve fibers that control blood vessel constriction in the legs. These thin, often unmyelinated nerves are part of the autonomic nervous system, the branch that operates below conscious awareness. When they’re damaged, the blood vessels in your lower limbs can’t tighten properly on standing, so blood pools instead of returning to the heart.
This is sometimes called neuropathic POTS, and it’s closely linked to small fiber neuropathy. Researchers have found that people with this form show decreased release of the signaling chemical norepinephrine in the lower extremities, even though their overall levels may look normal on a blood test. The nerve endings themselves are injured, so the signal to constrict simply doesn’t reach the blood vessels where it’s needed most. Small fiber neuropathy can also cause burning, tingling, or prickling sensations in the feet and legs, and it overlaps with conditions like chronic fatigue syndrome, fibromyalgia, and complex regional pain syndrome.
An Overactive Stress Response
An estimated 30 to 60% of POTS patients fall into the hyperadrenergic subtype, where the sympathetic nervous system (your “fight or flight” system) is essentially stuck in overdrive. Standing norepinephrine levels in these patients reach 600 pg/mL or higher, sometimes climbing past 1,000 pg/mL. For context, norepinephrine is the chemical your body releases during stress to speed up your heart and constrict blood vessels.
The result is a cluster of symptoms that feel a lot like a constant adrenaline surge: palpitations, tremors, anxiety, and a heart rate that spikes dramatically with minor position changes. Some people with this subtype also experience high blood pressure on standing rather than the low blood pressure seen in other forms, which can make the presentation confusing. The reason for the excessive sympathetic drive isn’t always clear, but autoimmune activity and problems with how norepinephrine is cleared from the bloodstream are both suspected contributors.
Low Blood Volume and a Hormonal Paradox
Many people with POTS are walking around with significantly less blood than they should have. A study published in Circulation found that POTS patients had an average total blood volume deficit of about 689 mL compared to controls. That’s roughly the equivalent of donating a pint and a half of blood.
What makes this especially puzzling is the hormonal picture. When blood volume drops, your body should respond by activating the renin-aldosterone system, a hormonal pathway that tells the kidneys to hold onto salt and water. But in POTS patients, renin activity was no different from healthy controls, and aldosterone levels were actually lower. This “renin-aldosterone paradox” means the body fails to mount the hormonal response that would normally correct the volume deficit. The low blood volume alone can explain the tachycardia: less blood returning to the heart means a lower stroke volume, which forces the heart to beat faster to maintain circulation.
Viral Infections as a Trigger
Many people trace the start of their POTS symptoms to a viral illness. COVID-19 has become the most prominent trigger, with estimates suggesting that 2% to 14% of COVID survivors develop POTS and as many as 61% experience POTS-like symptoms such as tachycardia, orthostatic intolerance, fatigue, and cognitive impairment within six to eight months of infection. These symptoms can persist well beyond the acute infection, overlapping heavily with what’s now called long COVID.
COVID-19 isn’t the only virus implicated. Epstein-Barr virus (the cause of mono), influenza, and other common infections have all been linked to POTS onset, though post-COVID cases appear to be more frequent than those following any other single virus. The proposed mechanisms include direct nerve damage from the infection, lingering inflammation, and an autoimmune response in which the immune system, activated to fight the virus, mistakenly targets the body’s own nervous system components.
Autoimmune Mechanisms
There’s growing evidence that POTS can be an autoimmune condition, at least in some patients. Researchers have found that up to 89% of POTS patients in studied cohorts had elevated levels of autoantibodies targeting a specific receptor on blood vessels and the heart (the alpha-1 adrenergic receptor), and about 53 to 56% also had autoantibodies targeting a receptor involved in the parasympathetic nervous system (the muscarinic M4 receptor). These antibodies can interfere with the normal signaling that controls heart rate and blood vessel tone.
Patients with these autoantibodies also showed abnormal levels of inflammatory markers in their blood, suggesting that ongoing immune activation plays a role in maintaining symptoms. This autoimmune connection helps explain why POTS so often develops after infections (which can trigger misdirected immune responses) and why it frequently coexists with other autoimmune conditions.
Connective Tissue Disorders
POTS is strikingly common in people with Ehlers-Danlos syndrome, particularly the hypermobile type. The connection comes down to the blood vessels themselves. In people with EDS, connective tissue throughout the body is more elastic than it should be. This includes the walls of veins and arteries. When those vessels are overly stretchy, they expand too easily under pressure, allowing blood to pool in the legs and abdomen instead of returning efficiently to the heart. The result is the same hemodynamic problem seen in other POTS subtypes: reduced blood return, lower stroke volume, and compensatory tachycardia.
Hypermobility spectrum disorder, a related condition that doesn’t meet the full diagnostic criteria for EDS, carries similar risks for abnormal autonomic regulation.
Mast Cell Activation
Some POTS patients have a coexisting condition called mast cell activation syndrome, in which immune cells called mast cells release histamine and other inflammatory chemicals inappropriately, often triggered by standing, exercise, or heat. These chemicals cause blood vessels to dilate, dropping blood pressure and provoking tachycardia. Patients with this overlap tend to have additional symptoms that look allergic in nature: flushing, nasal congestion, gastrointestinal problems, hives, and diarrhea.
Studies of young POTS patients have confirmed elevated histamine levels and other mast cell markers in a significant portion of those tested. This suggests that in some people, the tachycardia on standing isn’t purely a nervous system problem but also an inflammatory one.
Deconditioning and Other Contributing Factors
Physical deconditioning, essentially being out of shape due to prolonged bed rest or inactivity, can both cause and worsen POTS. When you’re inactive for weeks or months (after surgery, a long illness, or even a period of reduced movement), your heart shrinks slightly, blood volume drops, and your body becomes less efficient at managing position changes. This creates a vicious cycle: symptoms make you less active, inactivity worsens the cardiovascular deconditioning, and symptoms intensify.
Several other medical conditions can produce POTS or symptoms that mimic it. Diabetes can damage autonomic nerves over time. Alcoholism, heavy metal exposure, and certain chemotherapy drugs are also known to injure the small nerve fibers involved in blood vessel control. Before a POTS diagnosis is confirmed, other treatable causes of a fast heart rate on standing need to be ruled out, including thyroid disorders, adrenal gland problems, anemia, iron deficiency, and electrolyte imbalances.
Why Multiple Causes Often Overlap
One of the most important things to understand about POTS is that it rarely comes down to a single mechanism. A person might have mild small fiber neuropathy that becomes symptomatic only after a viral infection triggers autoantibody production and reduces their blood volume. Someone with hypermobile EDS might manage fine until deconditioning from an unrelated illness tips them over the edge. The combination of low blood volume, impaired nerve signaling, excessive sympathetic activation, and immune dysfunction creates a web of interacting problems, which is why POTS can be so difficult to diagnose and why treatment often needs to address multiple pathways at once.
Conditions that commonly travel alongside POTS reinforce this picture. Chronic fatigue syndrome, migraines, and post-concussion syndrome all show significant overlap with POTS, likely because they share underlying disruptions in autonomic regulation, inflammation, or both.