POTS stands for postural orthostatic tachycardia syndrome. The defining feature is a heart that speeds up quickly after you stand and stays fast, while blood pressure usually holds steady. That makes it a different pattern from the brief grey-out some people get from a blood pressure drop on standing, which has its own causes and its own workup.
Most standing symptoms have an everyday explanation, and the majority of those explanations never show up on a blood test. Short sleep, a recent illness with fluid loss, hot weather, alcohol the night before, long gaps between meals, a week spent mostly in bed, heavy menstrual bleeding, pregnancy, the postpartum months and ordinary life stress all make standing feel harder. A recently started or recently changed medication can do the same, which is one reason the prescribing clinician reviews your full list before anything else is considered.
Training load matters in both directions. A sudden jump in volume or intensity, especially in heat, leaves many people lightheaded and flat for days. So does the opposite. After surgery, a long illness or an extended stretch of bed rest, the heart and legs lose some of their ability to handle upright posture, and the first weeks back on your feet can feel awful even in a healthy person.
Then there is the specific pattern that defines POTS. Diagnostic criteria describe a sustained rise in heart rate of at least 30 beats per minute within 10 minutes of standing, occurring without a fall in blood pressure, with symptoms present for at least three months 1. In adolescents, the threshold used is a rise of at least 40 beats per minute 2. POTS is associated with onset after an infection, after surgery or after a period of prolonged immobility in a large share of cases 3.
The everyday causes and the clinical pattern are not mutually exclusive. Poor sleep, dehydration and heat make a genuine autonomic problem feel far worse, which is why a clinician asks about your week as carefully as they measure your pulse.
Blood work does not diagnose POTS. It is used to find or exclude the conditions that produce an almost identical set of symptoms, and reviews of the clinical approach describe a focused set of baseline tests used for exactly that purpose 4. Five markers come up repeatedly.
| Marker | Why it is linked to this symptom | What a result can and cannot tell you |
|---|---|---|
| TSH | Thyroid hormone influences resting heart rate, blood volume and heat tolerance, so an overactive or underactive thyroid can produce palpitations, tremor and fatigue | A clearly abnormal TSH points toward a thyroid cause worth following up. A normal TSH makes a thyroid explanation less likely but says nothing about your standing heart rate |
| Cortisol | Low adrenal output can cause fatigue, nausea, salt craving and lightheadedness on standing | A morning cortisol helps a clinician decide whether further adrenal assessment is needed. One reading is a screen, not an answer, and timing of the draw changes it |
| Sodium | Sodium tracks the balance between water and salt in the blood, and shifts in that balance affect how well you tolerate upright posture | A low or high sodium flags a fluid or electrolyte problem to investigate. A normal sodium does not rule out low blood volume |
| Hemoglobin | Anemia lowers oxygen delivery, and the heart compensates by beating faster, which feels like breathlessness and palpitations on standing | A low hemoglobin identifies anemia and prompts a search for the cause. A normal value does not exclude low iron stores |
| Ferritin | Ferritin reflects stored iron, and stores can run low before a blood count changes | A low ferritin points to depleted iron stores. Ferritin rises with inflammation, so a normal number can be falsely reassuring |
Typical lab reference ranges for these markers look roughly like the table below.
| Marker | Typical reference range | Units |
|---|---|---|
| TSH | 0.4 to 4.0 | mIU/L |
| Cortisol (morning, serum) | 6 to 23 | µg/dL |
| Sodium | 135 to 145 | mmol/L |
| Hemoglobin | 13.5 to 17.5 (men), 12.0 to 15.5 (women) | g/dL |
| Ferritin | 30 to 300 (men), 15 to 200 (women) | ng/mL |
These are typical lab reference ranges. Ranges vary by laboratory, age and sex, and your own report always takes priority over a general table.
It can, and that is why thyroid function is checked early. Thyroid hormone sets much of the background tone for heart rate, blood vessel behaviour and heat tolerance, so an overactive thyroid often brings palpitations, tremor, sweating, anxiety and weight change, while an underactive thyroid tends toward fatigue, cold intolerance and sluggishness. Both can make standing feel like work. Clinical guidance written for frontline practice describes thyroid function testing as part of the initial assessment when orthostatic symptoms are evaluated 5.
TSH is the usual first test because the pituitary adjusts it sensitively in response to thyroid hormone levels. A high TSH generally suggests an underactive thyroid and a low TSH an overactive one, though the free hormone levels are needed to interpret either properly.
Plenty of things move TSH without any thyroid disease being present. Acute illness, pregnancy, significant weight loss, time of day and several medications all shift the number. A single out-of-range TSH is a reason to repeat the test and look further, not a conclusion. Bring any abnormal result to a clinician who can see it alongside your symptoms and the rest of the panel.
Sometimes, because low adrenal output produces a symptom picture that overlaps heavily with standing intolerance. Fatigue that does not improve with rest, nausea, poor appetite, salt craving, weight loss and lightheadedness on standing can all appear when the adrenal glands are underproducing. Reviews of the diagnostic approach list adrenal insufficiency among the conditions considered before POTS is settled on 4.
Cortisol is usually measured in the morning, because output follows a daily rhythm that peaks shortly after waking and falls through the day. A morning value in the comfortable middle of the range makes adrenal insufficiency unlikely. A clearly low value is a screening finding that leads to a more specific assessment, not a diagnosis on its own.
Other things change the number. Shift work, acute illness, a badly timed draw, pregnancy and certain medications all alter what a single cortisol reading shows, which is why the clinician reviews your schedule and your medication list before reading it. A result outside the standard range is worth discussing with a clinician rather than acting on alone.
Sodium is checked because it reads the balance between salt and water in your blood, and that balance has a direct effect on how well you tolerate being upright. Low sodium can come with fatigue, headache, nausea, muscle cramps, poor concentration and lightheadedness, all of which overlap with the standing symptoms people describe. High sodium usually signals water loss.
There is a second reason it appears on the panel. Research suggests that many people with POTS have a lower circulating blood volume than expected, which is one of the features described in the condition 6. A routine Sodium result cannot measure blood volume directly, so a normal sodium does not rule that out. It simply tells the clinician that a gross electrolyte disturbance is not driving the picture.
Sodium moves for many ordinary reasons. Vomiting, diarrhoea, heavy sweating, drinking very large volumes of plain water quickly, kidney, heart or liver conditions and a number of medications all shift it. Any decision about fluids, salt or anything else based on that result belongs to the clinician who ordered it.
Yes, and it is one of the simplest explanations to check. Hemoglobin is the protein in red blood cells that carries oxygen. When it falls, tissues receive less oxygen per heartbeat and the heart compensates by beating faster, particularly when you stand and gravity pulls blood toward your legs. The result feels like breathlessness on stairs, a thumping pulse, pale skin, cold hands and a fog that does not lift. Anemia sits firmly on the list of conditions reviews describe as worth excluding before standing intolerance is attributed to anything else 1.
Hemoglobin comes as part of a complete blood count, and the red cell indices alongside it give the first clue about why it is low. Small, pale cells point one way, larger cells another.
The number also shifts for reasons unrelated to red cell production. Dehydration concentrates the blood and can make hemoglobin look better than it is, while pregnancy dilutes it. Altitude, smoking, recent blood donation, heavy periods and slow gastrointestinal blood loss all move it. A low hemoglobin always deserves a conversation about the cause rather than a conclusion drawn from the number alone.
It can, because iron stores run down long before a blood count changes. Ferritin is the storage form of iron, and a low ferritin with a normal hemoglobin is a common finding in people who feel exhausted, breathless on exertion, foggy and lightheaded without an obvious reason. People who menstruate heavily, endurance athletes and anyone with ongoing low-grade blood loss are the groups where it turns up most often.
Ferritin has one important quirk. It rises during inflammation, infection and liver stress, so a value in the normal range can mask depleted stores when something inflammatory is going on. Clinicians usually read it alongside an inflammatory marker for that reason, and they interpret a borderline value in the context of symptoms rather than as a simple pass or fail.
A low ferritin is a finding, not a plan. What to do about it, including whether any further testing is needed to find a source of blood loss, is a decision for a licensed clinician who has seen your full picture.
A completely normal panel is common in POTS, and it does not mean nothing is happening. POTS is identified by measuring heart rate and blood pressure during a standing or tilt assessment over time, not by blood work, so the labs serve to exclude other explanations rather than to confirm this one 4.
Several things that drive these symptoms leave no trace in a blood tube. Deconditioning after illness or immobility is one. Poor sleep quality, untreated sleep-disordered breathing, chronic stress, anxiety that coexists with genuine autonomic symptoms and simple under-fuelling are others. Joint hypermobility and gut motility problems frequently travel with standing intolerance and are found through examination and history, not serum chemistry.
The burden is also wider than the standing moment itself. Studies indicate that people with POTS report a heavy and varied symptom load including fatigue, cognitive difficulty, headache, sleep disturbance and gastrointestinal symptoms, much of it present even when seated 7. Research suggests that a subset of people with POTS carry autoantibodies and that autoimmune conditions appear alongside it more often than would be expected by chance 8. POTS is associated with a substantial reduction in quality of life across physical, social and occupational domains 9.
So the honest summary is that normal labs narrow the field. They do not close the question, and they are not a reason to stop looking.
Some symptoms need prompt attention rather than a scheduled appointment. Get urgent care for chest pain or pressure, for fainting that happens during exercise or while lying down, for fainting that causes a head injury, for a racing heart that will not settle after several minutes of rest, for sudden severe shortness of breath, or for new confusion or difficulty speaking.
Contact a clinician promptly for repeated fainting or near-fainting, for lightheadedness alongside very heavy menstrual bleeding or black or bloody stools, for standing symptoms that begin in pregnancy, and for symptoms that start within days of a change in medication. Rapid weight loss, persistent vomiting or a fever that is not resolving alongside these symptoms also warrants a prompt call.
None of this is a reason to diagnose yourself. Standing symptoms overlap across many conditions, and the pattern only becomes clear when someone measures your heart rate and blood pressure properly and reads your labs next to your history. A result outside the standard range is worth discussing with a clinician promptly rather than filing away.
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All five of these markers come from a single standard blood draw, and the FitScript Essential panel includes TSH, cortisol, sodium, hemoglobin and ferritin together, which is useful because they are far more informative read as a group than one at a time. Membership covers 150+ biomarkers tested per year, and Atlas, the AI health advisor, explains what each result means in plain language next to the standard reference range.
What testing will not do is tell you whether you have POTS. That takes a clinician measuring your heart rate response to standing over several minutes and comparing it against your symptom history. Labs sit underneath that assessment, clearing away the thyroid, adrenal, electrolyte and iron explanations so the remaining picture is easier to read. Any treatment decisions belong to a licensed provider who has reviewed you in person.
This article is health education and not medical advice. Discuss your own results and any next step with a clinician or licensed provider who knows your history.
Orthostatic intolerance is the general term for symptoms that appear when you stand up and ease when you lie down. It covers lightheadedness, palpitations, visual greying, shakiness, nausea and fatigue on standing. POTS is one specific pattern within it, defined by a sustained heart-rate rise rather than a fall in blood pressure. A clinician identifies which pattern applies.
No. In POTS, blood pressure usually holds steady while heart rate climbs sharply and stays elevated. A drop in blood pressure on standing is a different pattern with different causes. Both can feel like dizziness, so the distinction comes from measuring heart rate and blood pressure together during a standing assessment.
No blood test confirms POTS. Labs are used to find or exclude other conditions that cause the same symptoms, such as thyroid problems, low adrenal output, electrolyte disturbance, anemia and low iron stores. The diagnosis itself comes from measured heart rate and blood pressure responses to standing, interpreted by a clinician alongside your history.
It is the adult threshold used in the diagnostic criteria, measured as a sustained rise within 10 minutes of standing and occurring without a blood pressure drop. In adolescents a higher threshold is used. One home reading on a watch is not enough, because a clinician needs a properly timed measurement with symptoms considered too.
Thyroid hormone affects resting heart rate, heat tolerance and energy, so both an overactive and an underactive thyroid can produce palpitations, tremor and fatigue that mimic standing intolerance. TSH is the usual first screen. An abnormal value prompts further thyroid testing, while a normal value makes a thyroid explanation less likely.
Yes. Iron stores can be depleted well before hemoglobin falls, so you can feel exhausted, foggy and breathless with a normal blood count. Ferritin measures those stores. Because ferritin also rises with inflammation, a normal value is not always reassuring, which is why a clinician reads it alongside other markers.
The criteria require symptoms present for at least three months, alongside the measured heart-rate rise on standing. Shorter episodes are usually explained by something temporary such as illness, dehydration, heat or a period of bed rest. Keeping a simple log of symptoms, dates and pulse readings helps a clinician see the pattern.
Bring a log of when symptoms happen, how long they last and what you were doing, plus any heart rate readings taken lying down and after standing for several minutes. List every medication and supplement you take, note recent illnesses or surgeries, and bring any previous lab results. That history shapes which tests the clinician orders.