Understanding POTS, EDS, MCAS, and autoimmune conditions in the heat

If you live with endometriosis, you may already know that it rarely travels alone. Alongside a diagnosis of endometriosis, often arriving after years of being dismissed or misdiagnosed, many people find themselves accumulating further diagnoses: POTS, EDS, MCAS, lupus, thyroid disease. Conditions that are already complex on their own. Conditions that, in combination, create a body that functions by entirely different rules to the one most health advice assumes.

Summer amplifies all of it.

Heat is not a neutral backdrop for these conditions. It is an active trigger, a physiological challenge, a destabilising force. And yet the conversation around summer health rarely makes room for the particular experience of managing multiple invisible illnesses when the temperature rises.

This piece is an attempt to change that. To name what is happening in the body, to validate what people are already living through, and to offer some practical grounding for a season that can feel genuinely overwhelming, medically, socially, and emotionally.

You are not imagining it. The heat really does make it worse. All of it, often at the same time.

Why endometriosis and invisible illness so often coexist

For a long time, endometriosis was categorised primarily as a gynaecological condition. The growing body of research now positions it as systemic, involving the immune system, the nervous system, connective tissue, and inflammatory pathways throughout the body. This reframing helps explain something that patients have known anecdotally for years: that endometriosis tends to bring company.

Studies show that people with endometriosis are significantly more likely to be diagnosed with POTS, EDS, MCAS, and various autoimmune diseases than the general population. The mechanisms are still being researched, but current evidence points to shared immune dysregulation, connective tissue abnormalities, and mast cell pathways that may predispose the same body to multiple co-occurring conditions.

The connective tissue connection

Many of the conditions that most frequently co-occur with endometriosis, particularly hEDS, POTS, and MCAS, are thought to share an underlying connective tissue and mast cell dysregulation pathway. Some researchers have proposed that endometriosis may itself be influenced by connective tissue abnormalities, which would explain why these diagnoses cluster so consistently. In patient communities, this is sometimes called the hypermobile triad.

What this means practically is that if you have endometriosis and you also experience dizziness when standing, extreme fatigue that does not respond to rest, joints that dislocate or sublux, allergic-type reactions without clear cause, or symptoms that worsen dramatically in heat, you may not be experiencing separate and unrelated problems. You may be experiencing a body with multiple interconnected diagnoses, each of which needs to be understood in relation to the others.

The mechanics of heat: why warm weather is a physiological challenge

Before exploring how specific conditions interact with summer, it is worth understanding what is actually happening in the body when temperatures rise. For people managing multiple conditions, these processes do not happen in isolation. They stack.

Dehydration

Sweating causes the body to lose fluids and electrolytes rapidly, and even mild dehydration has measurable effects. It lowers the pain threshold, thickens the blood, and causes muscle cramping and headaches. For someone already managing endometriosis or a connective tissue condition, dehydration removes what little buffer existed between manageable and not manageable.

Inflammation

To cool the body, blood vessels near the skin dilate. This causes tissues, ligaments, and tendons around joints to expand and press on nerves, intensifying pain. High humidity significantly worsens inflammatory responses throughout the body. For conditions rooted in inflammation, including endometriosis, MCAS, and autoimmune disease, this is a direct physiological trigger, not a coincidence.

Nerve sensitivity

Hot weather dilates blood vessels and lowers blood pressure, which promotes fluid shifts and can irritate nerve endings, intensifying burning or tingling sensations. This helps explain why many people with conditions involving nerve involvement find that pain changes character in summer, becoming sharper, more electric, or more widespread.

Barometric pressure shifts

Heatwaves frequently coincide with drops in atmospheric pressure. This pressure change causes the fluids and tissues within joints to expand, leading to increased stiffness and pain. This is particularly relevant for people with hEDS and other connective tissue conditions, and it explains why symptom worsening often begins before the hottest part of the day, in the heavy, low-pressure air that precedes it.

When multiple conditions share a body, these four mechanisms do not each contribute a small amount of extra difficulty. They interact and amplify. The sum is considerably more than the parts.

POTS: when standing up becomes the hardest thing about summer

Postural Orthostatic Tachycardia Syndrome, known as POTS, is a condition affecting the autonomic nervous system. When a person with POTS moves from lying down to standing, their heart rate increases dramatically and their body struggles to maintain adequate blood pressure and circulation. The result is a cascade of symptoms: dizziness, near-fainting or fainting, heart palpitations, extreme fatigue, nausea, and brain fog.

For many people, POTS is a manageable background condition through cooler months. Summer changes this fundamentally.

Heat causes vasodilation: blood vessels widen, making it even harder for the POTS body to maintain blood pressure when standing. A warm afternoon that feels pleasant to most people can be genuinely incapacitating to someone with POTS.

This is why summer, with its outdoor events, its long days, and its cultural expectation of physical activity, is one of the most challenging seasons for people with POTS. The grocery run that was manageable in April can cause a fainting episode in July. Standing at a barbecue becomes impossible. Even sitting outside in the sun can trigger a significant symptom flare.

It is also why people with POTS are frequently dismissed or misunderstood. They may look completely well while their body is working at maximum capacity simply to remain upright. Many have been told their symptoms are anxiety, or that they just need to exercise more. The physiological reality is usually the precise opposite of what they have been advised.

Managing POTS through the summer months

• Cooling strategies before going outdoors: cooling vests, cold flannels applied to the neck and wrists, fans
• Electrolytes and salt loading: increasing salt intake (under medical supervision) expands blood volume and helps stabilise blood pressure
• Compression garments: graduated compression stockings and abdominal binders support venous return and are a medical aid, not a fashion choice
• Pacing: scheduling any physical activity for early morning when temperatures are lower, and planning rest before and after
• Avoiding heat sources: hot baths, saunas, standing in direct sun, and poorly ventilated spaces
• Medical alert identification for those at risk of loss of consciousness in public spaces

EDS: heat, fatigue, and the body that does not hold together

Hypermobile Ehlers-Danlos Syndrome (hEDS) is a connective tissue condition in which collagen, the structural protein that holds joints, skin, and organs together, does not function as it should. Joints are too flexible, frequently subluxing or dislocating. The body’s structural scaffolding is unreliable.

This might sound like it would be most problematic in cold weather. But heat presents its own distinct challenges for people with hEDS, and they are often underestimated.

Muscles do the stabilising work that connective tissue cannot. In heat, muscles fatigue faster. The result is that joints that were manageable in spring become significantly more unstable in summer, subluxing in response to activities that would not normally cause a problem. Carrying shopping bags. Walking on uneven ground at an outdoor festival. Sitting in a low camping chair. Activities that look entirely benign from the outside.

With hEDS, summer is not just about being hot. It is about being structurally less supported, and having no easy way to explain that to the people around you.

hEDS also frequently overlaps with POTS, meaning that many people are managing joint instability and dysautonomia simultaneously, both of which are worsened by heat, and both of which are invisible to everyone else on a warm summer day.

Managing hEDS through the summer months

• Joint support: braces, splints, and KT taping for commonly affected joints before any prolonged activity
• Mobility aids: using walking sticks, wheelchairs, or other aids without stigma, as they protect joints and conserve energy
• Activity planning: shorter bursts of activity with planned rest, avoiding prolonged standing, carrying, or repetitive movements
• Hydration: connective tissue and muscles need adequate hydration to function, and dehydration in heat worsens both joint stability and fatigue
• Rest access: identifying seating, rest areas, and cool spaces before attending any summer event

MCAS: when summer is full of triggers

Mast Cell Activation Syndrome (MCAS) is a condition in which mast cells, immune cells distributed throughout the body, release inflammatory mediators including histamine inappropriately and excessively. Symptoms range from flushing, hives, and itching to gastrointestinal pain and bloating, fatigue, brain fog, and in severe cases, anaphylaxis. For people with endometriosis, MCAS is a particularly common comorbidity, and some researchers believe there may be a shared mast cell pathway between the two conditions.

Summer is, in many ways, a worst-case scenario for MCAS. Not because of one trigger, but because of the sheer accumulation of them.

The summer trigger list for MCAS

Heat itself is a direct mast cell activator. Added to that: UV light and sun exposure, insect stings, pollen and grasses, chlorine in swimming pools, fragrances in sun creams, insect repellents, and summer skincare, and high-histamine foods that dominate summer social eating such as strawberries, tomatoes, alcohol, aged cheeses, and processed meats. Any one of these might be manageable. In combination across a single afternoon, the cumulative trigger load can cause a severe and unpredictable flare.

This makes summer social situations particularly complicated. A barbecue involves food triggers, heat triggers, insect sting risk, and fragrance exposure all at once. A day at the beach adds UV exposure and potentially chlorine. People with MCAS are frequently assumed to be difficult when they decline certain foods, avoid the sun, or leave early, when in fact they are making careful medical calculations about their own safety.

Managing MCAS through the summer months

• Sun protection: UV-protective clothing and fragrance-free, mineral-based sunscreen, as standard sunscreens are common fragrance triggers
• Dietary planning: identifying which summer foods are personal triggers and preparing your own food for picnics and barbecues where possible
• Antihistamine protocol: discuss with a specialist whether seasonal upweighting of antihistamines is appropriate
• Pool exposure: rinsing immediately after swimming and considering saltwater pools or avoiding chlorinated water
• Insect sting preparedness: epipen access and awareness of sting risk in outdoor settings
• Fragrance avoidance: checking labels on sun creams, insect repellents, and any outdoor products before use

Autoimmune conditions: the immune system in the heat

People with endometriosis are significantly more likely to develop autoimmune conditions, including lupus, rheumatoid arthritis, multiple sclerosis, Hashimoto’s thyroiditis, and inflammatory bowel disease among them. The connection is thought to involve shared immune dysregulation, though the precise mechanisms are still being researched.

Each of these conditions has its own relationship with summer, and for many people managing endometriosis alongside an autoimmune diagnosis, the season involves navigating multiple sets of triggers and constraints simultaneously.

Lupus (SLE): UV light is one of the most well-documented lupus triggers, capable of precipitating skin flares, profound fatigue, and systemic disease activity. People with lupus are generally advised to avoid direct sun exposure and use high-factor broad-spectrum sun protection, which creates a significant tension with the cultural expectation that summer means being outdoors.

Rheumatoid Arthritis: The relationship between RA and heat is variable. Some people experience temporary relief in warmth, while others find heat worsens inflammation. What is consistent is that prolonged physical exertion in summer can trigger post-exertional flares, and that the additional fatigue of heat compounds the existing fatigue of the condition.

Hashimoto’s Thyroiditis and Hypothyroidism: Thyroid dysfunction directly affects the body’s ability to regulate temperature. People with hypothyroidism often struggle disproportionately with heat, and the fatigue that is a primary symptom is significantly amplified in warm conditions. This is particularly difficult for people who are also managing endometriosis fatigue.

IBD (Crohn’s and Ulcerative Colitis): Dehydration is a significant IBD trigger and is more likely in summer. Outdoor social situations with limited toilet access, unfamiliar foods, and social pressure to eat and drink what everyone else is having create both practical and psychological challenges.

The compounding effect: when multiple conditions collide

The clinical literature tends to discuss these conditions in isolation. The lived experience is almost never isolated. A person managing endometriosis alongside POTS, hEDS, and MCAS is not managing four separate conditions that happen to share a body. They are managing a body in which these conditions interact, amplify each other, and collectively shape what any given day, and especially any given summer day, looks and feels like.

A warm day means blood pressure instability, joint instability, potential mast cell flares, fatigue amplification, and inflammatory pain, all at once. And then someone asks why you do not just come to the barbecue.

The social dimension of this compounding effect is significant and under-discussed. People managing multiple invisible conditions in summer face not just the physical symptoms but the social labour of explaining, justifying, and negotiating their limitations to people who cannot see them. The cumulative cost of always having to advocate for yourself in situations that are not designed for your body is a form of exhaustion that sits on top of everything else.

It also means that advice aimed at one condition often conflicts with management of another. The compression stockings recommended for POTS are hot, which is a problem for MCAS heat sensitivity. The increased activity recommended for POTS and EDS rehabilitation risks triggering post-exertional malaise in endometriosis. Managing this complexity requires specialist care that many people are still waiting to access.

You are not alone in this, and you are not imagining it

If you have read this piece and found yourself nodding, recognising your own summer in these descriptions, we want to be clear: the complexity you are living with is real, it is documented, and it is shared by a significant number of people who are also managing these conditions without adequate support or acknowledgement.

If you are still undiagnosed for one or more of these conditions, the journey to diagnosis for POTS, hEDS, MCAS, and autoimmune conditions, particularly for people who already have endometriosis, can be long and often requires advocacy. Keeping detailed symptom diaries, asking for specific referrals, and connecting with patient communities who can recommend informed specialists are steps that many people find invaluable.

You deserve care that sees the whole picture. Not just the endometriosis. Not just the POTS. All of it, together, in the context of a life that summer does not put on pause.

Key organisations and resources

Endometriosis South Coast: www.endometriosissouthcoast.com

POTS UK: potsuk.org

Ehlers-Danlos Support UK: ehlers-danlos.org

Mast Cell Action: mastcellaction.org

Lupus UK: lupusuk.org.uk

Crohn’s and Colitis UK: crohnsandcolitis.org.uk

The Samaritans: 116 123 (free, 24/7)

This blog post was written for the July Awareness Campaign. Medical claims are drawn from peer-reviewed literature; please see the accompanying Information Pack for a full reference list. This piece is for informational and advocacy purposes and does not constitute medical advice.