Weakness and fatigue are terms that are often used as if they mean the same thing, but in fact they describe two different sensations. It is important to know exactly what you mean when you say "I feel weak" or "I am fatigued" because it can help you and your doctor narrow down the possible causes of your symptoms.

• Weakness is a lack of physical or muscle strength and the feeling that extra effort is required to move your arms, legs, or other muscles. If muscle weakness is the result of pain, the person may be able to make muscles work, but it will hurt.
• Fatigue is a feeling of tiredness or exhaustion or a need to rest because of lack of energy or strength. Fatigue may result from overwork, poor sleep, worry, boredom, or lack of exercise. It is a symptom that may be caused by illness, medicine, or medical treatment such as chemotherapy. Anxiety or depression can also cause fatigue.

Both weakness and fatigue are symptoms, not diseases. Because these symptoms can be caused by many other health problems, the importance of weakness and fatigue can only be determined when other symptoms are evaluated.

Weakness

General weakness often occurs after you have done too much activity at one time, such as by taking an extra-long hike. You may feel weak and tired, or your muscles may be sore. These sensations usually go away within a few days.

In rare cases, generalized muscle weakness may be caused by another health problem, such as:

• Problems with the thyroid gland, which regulates the way the body uses energy.
  • A low thyroid level (hypothyroidism) can cause fatigue, weakness, lethargy, weight gain, depression, memory problems, constipation, dry skin, intolerance to cold, coarse and thinning hair, brittle nails, or a yellowish tint to the skin.
  • A high thyroid level (hyperthyroidism) can cause fatigue, weight loss, increased heart rate, intolerance to heat, sweating, irritability, anxiety, muscle weakness, and thyroid enlargement.
• Guillain-Barr� syndrome, a rare nerve disorder that causes weakness in the legs, arms, and other muscles and that can progress to complete paralysis.
• Myasthenia gravis, a rare, chronic disorder that causes weakness and rapid muscle fatigue.
• A problem with the minerals (electrolytes) found naturally in the body, such as low levels of potassium or sodium.

Muscle weakness that is slowly getting worse requires a visit to a doctor.

Sudden muscle weakness and loss of function in one area of the body can indicate a serious problem within the brain (such as a stroke or transient ischemic attack) or spinal cord or with a specific nerve in the body.

Fatigue

Fatigue is a feeling of tiredness, exhaustion, or lack of energy. You may feel mildly fatigued because of overwork, poor sleep, worry, boredom, or lack of exercise. Any illness, such as a cold or the flu, may cause fatigue, which usually goes away as the illness clears up. Most of the time, mild fatigue occurs with a health problem that will improve with home treatment and does not require a visit to a doctor.