Which diseases cause muscle weakness? A Medical Guide to Possible Causes

October 2, 2025 •

4 min read

Did you know that muscle weakness is one of the most common reasons for a doctor's visit, particularly among older adults? A vast number of conditions can present with this symptom, and understanding which diseases cause muscle weakness is the crucial first step toward finding the right diagnosis and treatment.

Quick Summary

Muscle weakness is a symptom of many conditions, including neurological disorders like multiple sclerosis and ALS, muscular dystrophies, autoimmune diseases such as myasthenia gravis, and endocrine imbalances affecting the thyroid or adrenal glands. Identifying the underlying cause is key to effective management.

Key Points

Neurological Problems: Conditions like MS, ALS, and nerve damage disrupt signals from the brain and spinal cord, causing muscle weakness.
Muscular Disorders: Myopathies, including muscular dystrophies and inflammatory diseases like polymyositis, directly damage muscle fibers, leading to weakness.
Autoimmune System Attacks: In diseases such as myasthenia gravis and lupus, the immune system mistakenly attacks healthy tissue, resulting in muscle function impairment and weakness.
Hormonal Imbalances: Endocrine issues like thyroid disorders (hypo- and hyperthyroidism) and adrenal gland conditions can significantly impact muscle strength and metabolism.
External and Metabolic Factors: Infections, certain medications (e.g., statins, corticosteroids), electrolyte imbalances, and chronic fatigue can also be significant causes of muscle weakness.
When to See a Doctor: Persistent or unexplained muscle weakness warrants medical evaluation, especially if it is severe, sudden, or accompanied by symptoms like difficulty breathing or numbness.

Neurological Conditions

Disorders of the nervous system can severely impair the communication between the brain, nerves, and muscles, leading to profound weakness. This can originate in the brain, spinal cord, or peripheral nerves.

Motor Neuron Diseases

Motor neuron diseases affect the nerve cells (motor neurons) that control voluntary muscle movement. As these cells deteriorate, the muscles they control weaken and waste away.

Amyotrophic Lateral Sclerosis (ALS): Also known as Lou Gehrig's disease, ALS is a progressive neurodegenerative disease that affects nerve cells in the brain and spinal cord, leading to the gradual loss of muscle control. Weakness typically begins in the limbs or with speech and swallowing, then spreads to other parts of the body.
Spinal Muscular Atrophy (SMA): A group of genetic diseases that destroy motor neurons, leading to muscle weakness and atrophy. The severity and age of onset vary widely depending on the type of SMA.

Autoimmune and Neuromuscular Junction Disorders

Some diseases cause the body's own immune system to mistakenly attack parts of the nervous system or the connection between nerves and muscles.

Multiple Sclerosis (MS): This autoimmune disease damages the myelin sheath that covers nerve fibers in the brain and spinal cord. The resulting poor nerve communication can cause muscle weakness, numbness, and coordination problems.
Myasthenia Gravis (MG): An autoimmune disorder that disrupts the communication between nerves and muscles at the neuromuscular junction. This leads to fluctuating muscle weakness that worsens with activity and improves with rest, commonly affecting eye and facial muscles first.
Guillain-Barré Syndrome (GBS): A rare disorder in which the body's immune system attacks its own peripheral nerves. It often follows an infection and causes rapid-onset muscle weakness and tingling, starting in the feet and legs and progressing upward.

Muscular Disorders (Myopathies)

Myopathies are diseases that directly affect the muscles themselves, causing them to become weak and dysfunctional. This group can be hereditary or acquired.

Muscular Dystrophies

This is a group of inherited genetic disorders that cause progressive muscle weakness and loss of muscle mass.

Duchenne and Becker Muscular Dystrophy: These are two of the most common types of muscular dystrophy, causing progressive weakness, primarily in the legs and hips.
Myotonic Dystrophy: A type of muscular dystrophy characterized by prolonged muscle contractions (myotonia) and progressive muscle weakness.

Inflammatory Myopathies

These autoimmune diseases cause muscle inflammation (myositis).

Polymyositis: Leads to muscle weakness, particularly in the proximal muscles (shoulders, hips, thighs).
Dermatomyositis: Similar to polymyositis but also causes a characteristic skin rash.
Inclusion Body Myositis (IBM): Typically affects older adults and causes progressive weakness and atrophy, often in the forearms and thighs.

Endocrine and Metabolic Diseases

Endocrine diseases affect hormone levels, which can significantly impact muscle function and strength. Metabolic disorders interfere with the body's energy production in muscle cells.

Endocrine Conditions

Thyroid Disorders: Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can lead to muscle weakness. In hyperthyroidism, it results from increased muscle breakdown, while in hypothyroidism, it's linked to metabolic slowdown.
Addison's Disease: This condition occurs when the adrenal glands produce too few hormones, leading to fatigue, generalized muscle weakness, and other symptoms.
Cushing's Syndrome: Caused by prolonged exposure to high levels of cortisol, this can lead to proximal muscle weakness and wasting.

Metabolic Conditions

Diabetes: Poorly managed blood sugar can lead to diabetic neuropathy, which damages the nerves controlling muscles, causing weakness and numbness.
Electrolyte Imbalances: Low levels of key electrolytes, such as potassium (hypokalemia) or magnesium (hypomagnesemia), can disrupt normal muscle and nerve function, causing weakness, cramps, or spasms.

Other Potential Causes

Infections: Viral infections like the flu, HIV, or Lyme disease can cause temporary or chronic muscle weakness. In rare cases, bacteria can directly infect the muscle (pyomyositis).
Toxins and Medications: Certain substances, both medicinal and recreational, can be toxic to muscle tissue. Common culprits include statin drugs used for cholesterol and prolonged use of corticosteroids. Excessive alcohol consumption can also lead to alcohol myopathy.
Chronic Fatigue Syndrome (CFS): A complex, chronic illness characterized by profound fatigue that is not relieved by rest and can include significant muscle weakness.
Cancer: Cancer-related muscle weakness, or cachexia, is common in advanced stages and can also be triggered by certain cancer treatments.

Distinguishing Different Causes of Muscle Weakness


Feature	Neurological Causes	Muscular (Myopathic) Causes	Endocrine Causes
Symptom Onset	Can be sudden (stroke) or gradual (ALS)	Usually gradual and progressive	Variable, often gradual
Location of Weakness	Variable, can be unilateral or bilateral. Can affect specific nerve distributions.	Often symmetrical and proximal (hips, shoulders)	Often generalized or proximal
Associated Symptoms	Numbness, tingling, balance issues, vision changes	Muscle cramps, pain, wasting (atrophy)	Fatigue, weight changes, mood changes, skin issues
Key Conditions	ALS, MS, Myasthenia Gravis	Muscular Dystrophy, Polymyositis	Thyroid disorders, Addison's disease

When to See a Doctor

If you experience persistent or unexplained muscle weakness, it is crucial to consult a healthcare professional. Seek immediate medical attention if muscle weakness comes on suddenly, is severe, or is accompanied by other urgent symptoms such as:

Difficulty breathing or swallowing
Sudden numbness or paralysis on one side of the body
Severe abdominal pain
A loss of consciousness

Conclusion

Muscle weakness is a broad symptom with a diverse range of underlying causes, from well-known neurological and muscular conditions to rarer metabolic and autoimmune disorders. While some instances may be temporary due to infection or electrolyte imbalance, persistent weakness requires medical evaluation to identify the root cause and begin appropriate treatment. Accurate diagnosis is essential for effective management and can be aided by a careful review of symptoms and medical history, laboratory tests, and specialized diagnostic procedures, as outlined by authoritative medical sources American Academy of Family Physicians.

Frequently Asked Questions

The most common causes can vary by age and overall health, but common culprits include infections like the flu, electrolyte imbalances, and aging-related muscle loss (sarcopenia). However, persistent or unusual weakness can indicate more serious issues.

Yes, deficiencies in certain vitamins, most notably Vitamin D and Vitamin B12, can cause muscle weakness. Correcting these deficiencies, often with supplements, can reverse the symptoms.

Seek immediate medical attention if muscle weakness is sudden, affects one side of the body, or is accompanied by difficulty breathing, swallowing, or vision changes. Persistent or progressive weakness should also be evaluated by a doctor.

Muscle weakness is a measurable lack of strength, where a muscle cannot contract normally. Fatigue, or asthenia, is a general feeling of weariness or exhaustion. A person with fatigue may still have normal muscle strength but find it takes more effort to use.

While stress and anxiety do not cause true muscle weakness, they can contribute to muscle tension, pain, and a feeling of general fatigue or exhaustion that may be perceived as weakness. This is sometimes called pseudowakness.

Yes, several medications can cause muscle weakness as a side effect. Common examples include statins (cholesterol-lowering drugs) and corticosteroids. If you suspect a medication is causing your symptoms, do not stop taking it without consulting your doctor.

Yes, a poor diet can contribute to muscle weakness through malnutrition, nutrient deficiencies (like vitamins or electrolytes), and other related health issues like anemia. A balanced diet is essential for muscle function.