Low blood pressure and fatigue often occur together and might be a sign of a serious medical condition. In some cases, hypotension, or low blood pressure, might be a direct cause of fatigue. In other cases, low blood pressure and fatigue are both symptoms of an underlying condition.

Hypotension occurs when blood pressure dips to an abnormally low level. The ranges of normal blood pressure vary from one individual to another, and no specific threshold defines low blood pressure, but some doctors use 90 systolic and 60 diastolic as a general benchmark. When either measurement falls below these markers, further investigation might be necessary.

Chronically low blood pressure is not, however, a cause for concern unless it is coupled with other symptoms. Low blood pressure and fatigue are a signal that there might be a more serious condition. Other common signs include an inability to concentrate, lightheadedness or dizziness, dehydration, depression, blurred vision, nausea, rapid breathing and cold or clammy skin. When low blood pressure and fatigue or any of these other symptoms are present, a visit to the doctor is recommended.

Causes of low blood pressure and fatigue range from the trivial to the life-threatening. Orthostatic or postural hypotension is caused by standing too quickly from a reclined position and can result in lightheadedness, blurred vision or possibly fainting. Similarly, prolonged bed rest can cause low blood pressure and fatigue. Dehydration reduces the blood’s volume, which reduces the blood pressure. Eating can draw a significant amount of blood to the stomach, again reducing the volume and pressure of blood in the major vessels.

Another possible cause of hypotension is pregnancy. During early pregnancy, a woman’s circulatory system extends to nourish the baby. Blood flowing through a longer network causes a reduction in pressure. During the first 24 weeks of pregnancy, blood pressure typically drops by about 10 points, then returns to previous levels after the woman gives birth.

Certain medications lower blood pressure as well. Antidepressants, beta blockers, diuretics, erectile dysfunction drugs and some Parkinson's disease medications have an effect on blood pressure. Narcotics and alcohol might amplify this effect.

Some causes of low blood pressure are much more serious. Shock, whether it is caused by blood loss, allergic reaction or septicemia, causes a severe, rapid drop in blood pressure and is life-threatening. Immediate emergency care is required for individuals who are going into shock or suffering severe trauma.

Some medical conditions are less obvious but also can be dangerous. Heart conditions such as bradycardia, an abnormally low heart rate, will circulate blood very slowly, reducing blood pressure. Thyroid conditions and other endocrine problems also can have an impact on blood pressure. Each of these conditions requires medical attention.