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 camera 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.

A stressful emotional situation may also cause fatigue. This type of fatigue usually clears up when the stress is relieved.

Many prescription and nonprescription medicines can cause weakness or fatigue. The use or abuse of alcohol, caffeine, or illegal drugs can cause fatigue.

A visit to a doctor usually is needed when fatigue occurs along with more serious symptoms, such as increased breathing problems, signs of a serious illness, abnormal bleeding, or unexplained weight loss or gain.

Fatigue that lasts longer than 2 weeks usually requires a visit to a doctor. This type of fatigue may be caused by a more serious health problem, such as:

* A decrease in the amount of oxygen-carrying substance (hemoglobin) found in red blood cells (anemia).
* Problems with the heart, such as coronary artery disease or heart failure, that limit the supply of oxygen-rich blood to the heart muscle or the rest of the body.
* Metabolic disorders, such as diabetes, in which sugar (glucose) remains in the blood rather than entering the body?s cells to be used for energy.
* 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.
* Kidney disease and liver disease, which cause fatigue when the concentration of certain chemicals in the blood builds up to toxic levels.