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 be determined only when other symptoms are evaluated. 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: A problem with the minerals (electrolytes) found naturally in the body, such as low levels of potassium or sodium. Infections, such as a urinary tract infection or a respiratory infection. 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-Barré 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. Muscle weakness that is slowly getting worse requires a visit to a doctor.