In medicine, a “mimic” is a condition whose symptoms so closely resemble another disease that it gets misdiagnosed. This happens more often than most people realize. Stroke mimics account for nearly 9% of all suspected stroke cases in emergency departments, and thyroid disorders masquerade as depression in roughly half of people with treatment-resistant depression. Understanding which conditions impersonate others can help you ask better questions, push for the right tests, and avoid unnecessary treatment.
Stroke Mimics
When someone arrives at an emergency department with sudden weakness, slurred speech, or facial drooping, the assumption is stroke. But in a study of over 1,500 suspected acute stroke cases, 8.8% turned out to be something else entirely. The most common culprits were seizures (20.4% of mimics), psychiatric conditions like severe anxiety (15.3%), low blood sugar (10.9%), and acute aortic dissection (9.5%).
Seizures can leave temporary weakness or confusion on one side of the body that looks identical to a stroke. Low blood sugar produces slurred speech and disorientation. Severe migraines with aura can cause numbness, vision changes, and difficulty speaking. Each of these requires a completely different response than a stroke, which is why brain imaging and blood glucose checks are standard when stroke is suspected.
Heart Attack vs. Heartburn and Panic Attacks
Chest pain is the hallmark of a heart attack, but it’s also the primary symptom of acid reflux. Both conditions can cause nausea, vomiting, and a burning sensation in the chest. This overlap makes acid reflux the most common cause of non-cardiac chest pain, and it’s one reason chest pain accounts for millions of emergency visits that turn out not to be heart-related.
Panic attacks add another layer of confusion. They produce chest tightness, shortness of breath, sweating, and a sense of impending doom, all of which overlap with heart attack symptoms. The key differences tend to be in timing and context. Heart attack pain often radiates to the arm, jaw, or back, worsens with exertion, and comes with a cold sweat. Acid reflux pain typically worsens after eating or when lying down, and responds to antacids. Panic attacks usually peak within 10 minutes and are often tied to a stressful trigger, though not always.
None of these distinctions are reliable enough for self-diagnosis. The overlap is precisely why emergency physicians run blood tests for heart-specific proteins and perform electrocardiograms rather than relying on symptom descriptions alone.
Thyroid Problems That Look Like Mental Illness
An underactive thyroid is one of the most overlooked medical mimics. It slows metabolism, which produces fatigue, weight gain, cold sensitivity, and dry skin. But it also causes forgetfulness, mental sluggishness, emotional instability, difficulty concentrating, and depression. In severe cases, it can trigger psychosis. About 50% of people with treatment-resistant depression have subclinical hypothyroidism, meaning their thyroid is underperforming just enough to affect brain chemistry without causing obvious physical symptoms.
An overactive thyroid mimics a different set of psychiatric conditions. It can produce agitation, anxiety, restlessness, and in older adults, severe psychosis or apathy. Graves’ disease, the most common cause of an overactive thyroid, frequently coexists with depression and anxiety disorders. Both overactive and underactive thyroid conditions can cause reversible dementia in older adults, meaning the cognitive decline disappears once thyroid levels are corrected.
Low levels of the thyroid hormone T3 are linked to reduced serotonin and noradrenaline, the same brain chemicals targeted by most antidepressants. This is why a simple blood test for thyroid function is so important before starting psychiatric medication. If the underlying cause is thyroid dysfunction, treating the thyroid resolves the psychiatric symptoms.
Conditions That Mimic Multiple Sclerosis
Multiple sclerosis produces nerve damage that causes numbness, tingling, vision problems, weakness, and difficulty walking. The trouble is that a long list of other conditions does the same thing. Vitamin B12 deficiency can cause progressive leg weakness and spinal cord changes that look like MS on an MRI, though B12-related damage tends to appear symmetrically in a specific part of the spinal cord. Lyme disease can produce facial paralysis, widespread tingling from nerve root involvement, and spinal cord symptoms that closely resemble an MS flare.
Lupus can present with a relapsing-remitting pattern nearly indistinguishable from MS, where symptoms flare and then partially resolve over time. Other autoimmune conditions like Sjögren’s syndrome, Behçet’s disease, and sarcoidosis can also produce multiple brain or spinal cord lesions that mimic MS on imaging. Differentiating these conditions usually requires blood work looking for specific inflammatory markers, antibodies, and sometimes a spinal tap.
Conditions That Mimic Lupus
Lupus itself is notoriously difficult to diagnose because it can affect nearly any organ system, producing joint pain, rashes, fatigue, kidney problems, and neurological symptoms. But several conditions produce a convincingly similar picture. Viral infections, particularly parvovirus B19, hepatitis, HIV, and Epstein-Barr virus, can trigger joint pain, rashes, and blood test abnormalities that overlap with lupus. Certain cancers, especially lymphomas, can cause fevers, weight loss, joint inflammation, and even positive blood tests for the same antibodies seen in lupus.
Other autoimmune diseases like dermatomyositis, Sjögren’s disease, and mixed connective tissue disease share enough features with lupus to create diagnostic confusion. Rare genetic immune disorders can also produce a near-perfect lupus imitation, sometimes discovered only after standard lupus treatments fail to work. In one documented case, a patient with joint inflammation and blood abnormalities typical of lupus was eventually found to have a genetic immune deficiency, identified only through specialized genetic testing after conventional treatments had no effect.
Seizures vs. Fainting vs. Psychogenic Episodes
When someone collapses and their body jerks, it looks like a seizure. But fainting (syncope) can produce brief tonic or clonic movements that mimic a convulsion. Psychogenic nonepileptic episodes, sometimes called PNES, look like seizures but have no abnormal electrical activity in the brain. These episodes are real and involuntary, but they stem from psychological processes rather than the misfiring neurons that cause epileptic seizures.
Distinguishing between these requires careful observation and testing. Two physical signs that moderately favor a psychogenic episode over epilepsy are closed eyes during the event and limb movements that appear asynchronous rather than rhythmic. People with psychogenic episodes more commonly report headache and dizziness before an event, while people with epilepsy more often have seizures after missing their medication. After a true epileptic convulsion, blood levels of the hormone prolactin typically rise, while they stay normal after a psychogenic episode. The gold standard for diagnosis is video EEG monitoring, where brain activity is recorded during a typical episode. In psychogenic episodes, the EEG shows no seizure activity even while the patient is visibly convulsing.
Pulmonary Embolism Mimics
A blood clot in the lungs causes chest pain that worsens with breathing, shortness of breath, and sometimes a rapid heart rate. These symptoms overlap heavily with pleurisy (inflammation of the lung lining), pneumonia, and even severe anxiety. In one case report, a patient presented with chest pain, vomiting, diarrhea, and respiratory distress that initially suggested a lung infection, but CT imaging revealed blood clots blocking the pulmonary arteries.
The danger of this mimicry runs in both directions. Pleurisy and pneumonia can be mistaken for a pulmonary embolism, leading to unnecessary blood thinners. And a pulmonary embolism can be dismissed as a simple infection, delaying lifesaving treatment. CT scans with contrast dye are the standard way to tell these apart, which is why doctors order imaging quickly when the symptoms are ambiguous.
Urinary Tract Infections and Delirium
In adults over 55, a urinary tract infection can produce sudden confusion, agitation, hallucinations, or dramatic personality changes that look like the onset of dementia or a psychiatric emergency. UTIs are one of the most common bacterial infections in older adults, and they’re strongly linked to the onset or worsening of neuropsychiatric symptoms. A previously sharp older adult who becomes suddenly confused and disoriented may not need a psychiatrist or neurologist. They may need a simple urine test.
Why Mimics Matter
Diagnostic errors from medical mimics lead to longer hospital stays, more doctor visits, and inappropriate treatments. A person treated for depression when their actual problem is an underactive thyroid may spend years on antidepressants that never fully work. Someone treated for epilepsy when they have psychogenic episodes will take seizure medications that carry real side effects but address nothing. The pattern holds across medicine: treating the mimic instead of finding the true cause delays recovery and sometimes causes harm. Knowing that these overlaps exist gives you a practical edge in understanding your own symptoms and the questions worth raising with your doctor.