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?
Medical Disclaimer: For educational purposes only. Always consult a qualified healthcare professional for diagnosis and treatment.