Symptom Guide

Muscle Weakness

Name: Muscle Weakness: Causes & Blood Tests | HealthReview
Creator: Suman Konda
Published: 2026-07-05

Muscle weakness has many causes, from simple electrolyte deficiencies to thyroid disease, myopathy and nerve damage. Blood tests identify most metabolic causes.¹

Written by Suman Konda, PharmD, Clinical Pharmacist · Editorial policy · Not medical advice

Last updated: 01 August 2026 · How we check our content

Blood tests ordered for muscle weakness

Test	Normal Range	What weakness it detects
TSH	0.4–4.0 mIU/L	Hypothyroidism: proximal muscle weakness
Vitamin D (25-OH)	30–100 ng/mL	Deficiency causes muscle pain and weakness
Potassium	3.5–5.0 mEq/L	Hypokalaemia: classic cause of muscle weakness
Calcium	8.5–10.5 mg/dL	Hypocalcaemia: muscle cramps; hypercalcaemia, weakness
CK (Creatine Kinase)	Men: 55–170 U/L; Women: 30–135 U/L	Raised in myopathy, myositis, rhabdomyolysis
Aldolase	1.0–7.5 U/L	Inflammatory myopathy (polymyositis/dermatomyositis)
Cortisol	AM: 6–23 mcg/dL	Adrenal insufficiency: fatigue and weakness

What does a high CK mean with muscle weakness?

Creatine kinase and muscle damage

CK is an enzyme inside muscle cells. When muscle is damaged or inflamed, CK leaks into the blood. Mild-moderate elevation (2–10x normal): statin-induced myopathy, hypothyroidism, polymyositis. Marked elevation (>10x normal): rhabdomyolysis (muscle breakdown), can cause acute kidney failure from myoglobin in the urine. Very high CK with brown urine and muscle pain is a medical emergency.

Causes of proximal muscle weakness

Cause	Key clue
Hypothyroidism	TSH raised; other thyroid symptoms
Vitamin D deficiency	25-OH D <20 ng/mL; bone pain too
Polymyositis	CK very high; anti-Jo1 antibody positive
Statin myopathy	On statin medication; CK mildly raised
Cushing's syndrome	High cortisol; central obesity, easy bruising
Hypokalaemia	Potassium <3.5; often with fatigue

Questions to ask your doctor

Is my CK raised: could I have a muscle disease?
Is my weakness from thyroid, Vitamin D, or electrolytes?
Could my statin be causing muscle pain and weakness?
Do I need a nerve conduction study or EMG?

Frequently Asked Questions

What's the difference between tiredness and true muscle weakness?

True weakness means the muscle genuinely can't generate normal force (e.g. difficulty rising from a chair or lifting objects), whereas fatigue is a sense of exhaustion with preserved strength. The distinction guides which tests are needed.

Which blood tests investigate muscle weakness?

Depending on the pattern, doctors may check electrolytes (potassium, calcium, magnesium), thyroid function, vitamin D, creatine kinase (CK), and glucose: each screens for a specific reversible cause.

When should muscle weakness be treated as urgent?

Sudden weakness on one side, difficulty breathing or swallowing, or rapidly spreading weakness needs emergency assessment, as these can signal stroke or a serious neurological condition.

References

Sources cited on this page. PubMed links open the original abstract.

Barohn RJ, Dimachkie MM, Jackson CE. A pattern recognition approach to patients with a suspected myopathy. Neurol Clin. 2014;32(3):569–593. PMID 25037082 · doi:10.1016/j.ncl.2014.04.008

Upper versus lower motor neuron weakness – a key clinical distinction

Neurological muscle weakness is classified by where in the nervous system the problem lies:

Upper motor neuron (UMN) lesion (brain or spinal cord): Weakness with increased muscle tone (spasticity), brisk reflexes, and an upgoing plantar reflex (Babinski sign). Causes include stroke, multiple sclerosis, and spinal cord compression. Blood tests are often normal; MRI is the primary investigation.
Lower motor neuron (LMN) lesion (nerve or neuromuscular junction): Weakness with reduced tone (flaccidity), diminished reflexes, and wasting. Causes include peripheral neuropathy, Guillain-Barré syndrome, and motor neuron disease. Nerve conduction studies and EMG are diagnostic.
Muscle disease (myopathy): Weakness with normal or reduced reflexes, no sensory loss, typically proximal (difficulty rising from a chair or raising arms above head rather than fine hand weakness). Creatine kinase (CK) is often markedly elevated.

Blood tests that evaluate muscle weakness

When muscle weakness cannot be explained by a simple physical examination finding, blood tests identify treatable systemic causes:

Creatine kinase (CK): The most sensitive marker of muscle cell damage. Elevated in inflammatory myopathies (dermatomyositis, polymyositis), rhabdomyolysis, muscular dystrophies, statin-induced myopathy, and hypothyroid myopathy. A CK more than 10 times the upper limit of normal is the threshold for considering inflammatory myopathy or toxic myopathy.
Thyroid function (TSH): Both hypothyroidism and hyperthyroidism cause muscle weakness. Hypothyroid myopathy is characterised by proximal weakness, myalgia, delayed relaxation of reflexes, and a raised CK. It reverses completely with thyroxine replacement.
Potassium and magnesium: Hypokalaemia (low potassium) and hypomagnesaemia cause profound muscle weakness and cramps. Causes include diuretic therapy, vomiting, diarrhoea, and primary hyperaldosteronism. Periodic paralysis (rare) is caused by episodic potassium shifts into cells.
Calcium: Hypercalcaemia causes generalised weakness, fatigue, and confusion ("bones, groans, stones, and moans"). Hypocalcaemia causes muscle cramps and tetany (carpopedal spasm).
ANA and myositis-specific antibodies (anti-Jo-1, anti-Mi-2, anti-MDA5): Autoimmune markers for inflammatory muscle diseases. Anti-Jo-1 in particular indicates antisynthetase syndrome – inflammatory myopathy with interstitial lung disease and a specific treatment approach.
Aldolase: Another muscle enzyme, sometimes elevated in inflammatory myopathy when CK is only mildly raised.
Vitamin D: Severe vitamin D deficiency causes proximal myopathy that closely mimics inflammatory muscle disease and is fully reversible with supplementation.