Several autoimmune diseases can cause heat intolerance, though none of them owns the symptom. An overactive thyroid and multiple sclerosis are the best known, and Sjögren’s syndrome, autoimmune nerve conditions, and Addison’s disease can do it too. They get there in very different ways: some make the body produce too much heat, some damage the nerves or glands that cool you, and some let heat expose weak spots in damaged nerves. That’s why the other symptoms you have alongside the heat trouble usually say far more about the cause than the heat itself.
How Your Body Normally Handles Heat
When your body warms up, nerves open the blood vessels in your skin so heat can escape, and they switch on sweating so evaporation cools you.1PubMed. Responses to hyperthermia. Optimizing heat dissipation by convection and evaporation: Neural control of skin blood flow and sweating in humans This widening system has no resting activity. It turns on only when your body temperature rises, in hot weather or during exercise.2PubMed Central. Mechanisms and modifiers of reflex induced cutaneous vasodilation and vasoconstriction in humans In the cold, a separate set of nerves narrows the skin vessels to keep heat in.3PubMed Central. Mechanisms and modifiers of reflex induced cutaneous vasodilation and vasoconstriction in humans So anything that damages these nerves or the sweat glands, or that speeds up how much heat your body makes, can leave you overheating in conditions other people tolerate.
Feeling hot is also not the same as having a fever. Many people with autoimmune disease feel overheated while the thermometer reads normal. A true fever comes from inflammation chemicals acting on the brain’s temperature setting, and checking your temperature is the quick way to tell the two apart. Fitness, hydration, age, and how used to the heat you are all change how well you cool, so a change from how you used to cope is more telling than any comparison with other people.
Graves’ Disease and Other Overactive Thyroid Conditions
Heat intolerance is a well-known symptom of Graves’ disease, an autoimmune condition where antibodies push the thyroid to overproduce hormone.4PubMed. Seasonal variation in the diagnosis of Graves’ disease People often feel hot, sweaty, and restless when everyone else is comfortable. Other common clues include weight loss despite a big appetite, a racing heartbeat, shaky hands, and sometimes bulging eyes.
The eye changes can include a staring look, gritty or watery eyes, or swelling around the eyes, because the same antibodies affect tissue behind the eye. They can appear even when thyroid hormone levels are only mildly raised or are being treated, so they need their own check.
Ordinary summer discomfort fades when the weather cools. Thyroid-driven heat intolerance tends to persist in air-conditioned rooms and cold weather too. Feeling warm all the time, with a fast pulse at rest and trouble sleeping, points toward the thyroid rather than the weather.
Hashimoto’s thyroiditis usually ends in an underactive thyroid, but it can start with an overactive phase that typically lasts one to two months.5PubMed Central. Prolonged Duration of Hashitoxicosis in a Patient with Hashimoto’s Thyroiditis: A Case Report and Review of Literature In that phase a person can have increased appetite, heat intolerance, fatigue, and sweating, and in one reported case it lasted two years.6PubMed Central. Prolonged Duration of Hashitoxicosis in a Patient with Hashimoto’s Thyroiditis: A Case Report and Review of Literature This phase can be mistaken for Graves’ disease, because both show a very low TSH (thyroid-stimulating hormone, the hormone that drives the thyroid) and high thyroid hormone.7PubMed Central. Clinical Case Series of Destructive Thyrotoxicosis Associated with Hashimoto’s Thyroiditis Misdiagnosed as Graves’ Disease: Clinical Patterns, Diagnostic Pitfalls, and Hypothesized Molecular Insights A thyroid uptake scan helps, since the Hashimoto’s phase shows low uptake, pointing to a leaking, damaged gland rather than an overworking one.8PubMed Central. Clinical Case Series of Destructive Thyrotoxicosis Associated with Hashimoto’s Thyroiditis Misdiagnosed as Graves’ Disease: Clinical Patterns, Diagnostic Pitfalls, and Hypothesized Molecular Insights
The first blood test is simple. A doctor usually orders TSH and free T4 first, and a low TSH with a high free T4 means the thyroid is overactive. Antibody tests then help separate Graves’ from Hashimoto’s, which matters because the treatment differs.
Multiple Sclerosis and Neuromyelitis Optica
In MS, the protective coating on nerves is damaged, and warming changes how those damaged nerves carry electrical signals, so they slow or fail.9PubMed Central. Uhthoff`s phenomenon 125 years later – what do we know today? Symptoms like blurred vision, weakness, or numbness can appear or worsen in a hot shower, hot weather, or exercise, then ease after cooling. A heat flare usually isn’t new permanent damage.
Heat sensitivity is common in both MS and neuromyelitis optica. About half of people with MS, and a similar share with neuromyelitis optica, report worse symptoms when body temperature rises.10European Neurology. Uhthoff’s Phenomenon in Multiple Sclerosis and Neuromyelitis Optica Weakness and sensory changes were more common than vision changes in both.11European Neurology. Uhthoff’s Phenomenon in Multiple Sclerosis and Neuromyelitis Optica
Hot showers and baths are a classic trigger, which is why some people with MS test bath water with a thermometer or keep it lukewarm. Fever from an infection, exercise, saunas, hot tubs, and sun exposure raise body temperature and can set off the same flare.
A heat flare is sometimes called a pseudo-relapse: a temporary return of old symptoms caused by raised body temperature rather than new inflammation in the nervous system. A true relapse involves fresh inflammation, lasts longer, and can leave lasting changes, so the two are treated very differently. Recognizing a pseudo-relapse can spare you steroids you don’t need, though a doctor should make that call. Symptoms that last more than a day after you’ve cooled down may signal a real relapse, so call your neurologist.
Sjögren’s Syndrome and Loss of Sweating
Sjögren’s is known for dry eyes and dry mouth, but people with it also sweat less than others.12PubMed. Impaired sweating as an exocrine manifestation in Sjögren’s syndrome In one reported case, heat intolerance from a total loss of sweating was the main sign of Sjögren’s.13PubMed. A case of Sjögren’s syndrome with heat intolerance induced by generalized anhidrosis as its principal feature The sweat glands looked normal, yet they did not respond to nerve signals, which suggests antibodies block the signal.14PubMed. A case of Sjögren’s syndrome with heat intolerance induced by generalized anhidrosis as its principal feature
When sweating has stopped, skin stays dry and flushed during exercise or heat. You may feel faint and hot, and get headaches quickly. Patchy sweating can be another clue, and dry eyes and mouth alongside it point toward Sjögren’s.
Not sweating is risky because the body’s main cooling route is gone, and heat exhaustion can come faster than expected. Cooling sprays or wet cloths can stand in for sweat, so plan to have them with you.
Doctors diagnose Sjögren’s by combining symptoms with blood tests for specific antibodies, eye tests for tear production, and sometimes a small biopsy of salivary gland tissue. Mentioning sweating changes to your doctor adds useful information.
POTS, Autoimmune Dysautonomia, and Nerve Damage
POTS is a condition where heart rate jumps on standing, with dizziness and fatigue. In one group with autoimmune POTS, about 4 in 10 reported heat intolerance or inability to sweat.15PubMed Central. Autoimmune postural orthostatic tachycardia syndrome Many people with POTS carry antibodies against nerve and blood-vessel receptors.16PubMed Central. Postural Orthostatic Tachycardia Syndrome, Myalgic Encephalomyelitis/Chronic Fatigue Syndrome and Long COVID as Neuroimmune Disorders Heat widens blood vessels, so it can worsen dizziness in POTS.
Symptoms often ease when you lie flat, which can be one clue toward POTS among other causes of fatigue. Heat, large meals, and long periods of standing tend to make episodes worse, so many people plan around them.
A standard test measures heart rate and blood pressure lying down and then standing, looking for a sustained rise in heart rate without a big blood pressure drop. Doctors also rule out other causes of a fast heart rate, such as anemia, thyroid problems, dehydration, and medicines. An overactive thyroid can mimic POTS, so a thyroid blood test is a usual early step.
Damage to the small nerves that control automatic functions can also disrupt temperature control.17PubMed. Thermoregulation in neuropathies Causes include immune-mediated autonomic neuropathy, as well as diabetes.18PubMed. Thermoregulation in neuropathies For severe autoimmune dysautonomia not helped by other treatment, immune therapy given by drip helped about 8 in 10 in one group, with first improvement around 5 weeks.19PubMed. Intravenous Immunoglobulin Therapy in Refractory Autoimmune Dysautonomias: A Retrospective Analysis of 38 Patients
Several clues point toward dysautonomia: dizziness or a racing heart when standing, feeling worse after hot showers or standing in heat, bladder, gut, or vision changes along with the heat trouble, and symptoms that began after a viral illness, including COVID.
Addison’s Disease and Heat
Autoimmune Addison’s destroys the adrenal glands, which make cortisol and aldosterone. People with it lack aldosterone and are more prone to dehydration and salt imbalance.20Addison’s Disease Self-Help Group. Heat & Hydration Heat stress tends to bring fatigue and weakness.21Addison Disease Society of Canada. Managing Heat with Adrenal Insufficiency and Thyroid Disorders
Aldosterone tells the kidneys to hold on to salt and water, so low levels mean more of both are lost. Heat makes this worse because sweating removes still more salt and water.
In heat, salt craving, dizziness, and nausea can signal trouble. Vomiting or confusion may mean an adrenal crisis, which needs emergency care. Doctors may advise extra fluids, extra salt, or adjusting steroid doses in hot weather.
Addison’s usually develops gradually, with tiredness, weight loss, poor appetite, and darkened skin, particularly in creases and scars. Heat sensitivity is a fairly nonspecific clue here, so the other symptoms matter more.
Medicines That Can Add to Heat Intolerance
Many long-term medicines change how well the body sheds heat, adding to strain during hot weather or exercise.22PubMed Central. Effects of Medications on Heat Loss Capacity in Chronic Disease Patients: Health Implications Amidst Global Warming If your heat intolerance started with a new drug, mention it to your doctor.
Water pills increase fluid loss. Some antidepressants and bladder drugs reduce sweating. Beta-blockers limit the heart’s response to heat. Never stop a medicine without asking first. If your body already struggles to cool because of nerve damage or low sweating, a drug that trims cooling further can tip you over.
Getting Checked and When to See a Doctor
If you feel overheated, take your temperature, since a fever could mean infection.23CreakyJoints. Feeling Hot and Overheated from Inflammatory Arthritis: Why It Happens and How to Cope If sweating seems reduced, a test that measures sweat produced by small nerves is the most widely used check of sweating nerves.24PubMed. Assessment of sudomotor function A whole-body sweat test maps where you sweat after warming, showing where nerve damage lies.25PubMed. Assessment of sudomotor function
A primary care doctor is the usual first stop and can order the temperature check, thyroid blood tests, and a medicine review. If those are normal and you have nerve-type symptoms, they may refer you to a neurologist or autonomic specialist. Dry eyes and mouth with joint pain may lead to a rheumatologist.
Beyond thyroid tests, doctors may check blood count, blood sugar, kidney function, sodium and potassium, and inflammation markers. If symptoms fit, they may add antibody tests for conditions like Sjögren’s or Addison’s.
Bring a log of when heat symptoms happen, other symptoms such as weight change, numbness, dry eyes, or dizziness, any family history of thyroid or autoimmune disease, and a list of all your medicines.
Confusion, fainting, or a very high temperature need emergency care. Hot, dry skin in heat can be a danger sign of heatstroke, though people with heatstroke can also still be sweating.
Ways to Stay Comfortable in the Heat
Cooling garments improve walking, mobility, and fatigue in people with MS, in warm and mild conditions.26PubMed. The effect of cooling garments to improve physical function in people with multiple sclerosis: A systematic review and meta-analysis In one test, a cooling vest let heat-sensitive people with MS walk about 30% longer.27PubMed. Effects of a cooling vest with sham condition on walking capacity in heat-sensitive people with Multiple Sclerosis Children overheat more easily, so kids with autoimmune conditions need extra care in hot weather.28Journal of Preventive and Complementary Medicine. Climate change as a determinant of pediatric autoimmune disease in Juvenile Idiopathic Arthritis
Vests, neck wraps, and cooling bandanas are common options, some using ice packs and others using evaporation or other cooling materials. Ice-pack styles cool strongly but wear off in time, so people often carry spare packs. Evaporative ones suit dry climates better than humid ones, where moisture doesn’t evaporate well.
Daily habits help too. Exercise in the cool morning or evening. Use fans, cool showers, and wet cloths on your neck and wrists. Drink fluids steadily, and ask about salt if you have Addison’s. Wear loose, light clothing and plan shaded breaks.
Pre-cooling, such as a cool drink or cool shower before going out or exercising, gives you a lower starting temperature and more time before symptoms begin. This is especially useful for people with MS, who can often tell roughly how much heat they can take before a flare starts.