[General Internal Medicine] What are the possible causes of lower back pain, leg fatigue, and generalized weakness?

Generalized fatigue, low back pain, and leg discomfort—often described as “aching” or “heaviness”—can stem from a wide range of underlying causes, spanning musculoskeletal, neurological, endocrine, hematologic, infectious, and psychiatric domains. A thorough clinical evaluation is essential to determine the root cause.

Musculoskeletal contributors are common and include chronic lumbar strain, degenerative disc disease, spinal stenosis, or myofascial pain syndrome—particularly if symptoms worsen with prolonged standing, sitting, or physical exertion and improve with rest or positional change. Vitamin D deficiency or electrolyte imbalances (e.g., hypokalemia, hypomagnesemia) may also manifest as diffuse muscle aching and profound fatigue.

Systemic conditions must be carefully considered. Hypothyroidism frequently presents with fatigue, muscle stiffness, and non-specific back or limb discomfort. Anemia—especially iron-deficiency or vitamin B12–folate deficiency anemia—can produce generalized weakness, lassitude, and exercise intolerance. Chronic inflammatory conditions such as rheumatoid arthritis or ankylosing spondylitis may involve axial and peripheral musculoskeletal symptoms alongside systemic fatigue.

Neurological etiologies include lumbar radiculopathy (e.g., from herniated nucleus pulposus), peripheral neuropathy (e.g., due to diabetes, alcohol use, or autoimmune causes), or, less commonly, early presentations of neuromuscular disorders like myasthenia gravis or inflammatory myopathies. Sleep disorders—particularly obstructive sleep apnea—can lead to non-restorative sleep, resulting in persistent fatigue and vague musculoskeletal complaints.

Psychiatric factors—including major depressive disorder and generalized anxiety disorder—are well-recognized causes of somatic symptom presentation, often featuring fatigue, unexplained aches, and reduced energy without objective physical findings. Medication side effects (e.g., statins, antihypertensives, antidepressants) and chronic low-grade infections (e.g., latent tuberculosis, chronic viral hepatitis) should also remain on the differential.

A targeted history—including symptom onset, duration, diurnal variation, aggravating/relieving factors, associated features (e.g., fever, weight loss, night sweats, paresthesias, bowel/bladder changes)—along with a focused physical examination and judicious laboratory testing (e.g., CBC, comprehensive metabolic panel, TSH, vitamin D, ferritin, HbA1c, ESR/CRP) is critical. Imaging (e.g., lumbar spine MRI) or electrophysiologic studies may be warranted based on clinical suspicion. Early referral to appropriate specialists—such as rheumatology, neurology, or endocrinology—is indicated when red flags are present or initial evaluation is inconclusive.