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.

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.