Many factors can cause thrombocytopenia (a low platelet count). The condition can be inherited or acquired. "Inherited" means your parents pass the gene for the condition to you. "Acquired" means you aren't born with the condition, but you develop it. Sometimes the cause of thrombocytopenia isn't known.
                
In general, a low platelet count occurs because:
                
The body's bone marrow doesn't make enough platelets.
                
The bone marrow makes enough platelets, but the body destroys them or uses them up.
                
The spleen holds on to too many platelets.
                
A combination of the above factors also may cause a low platelet count.
                
The Bone Marrow Doesn't Make Enough Platelets
                
Bone marrow is the sponge-like tissue inside the bones. It contains stem cells that develop into red blood cells, white blood cells, and platelets. When stem cells are damaged, they don't grow into healthy blood cells.
                
Many conditions and factors can damage stem cells.
                
Cancer
                
Cancer, such as leukemia (lu-KE-me-ah) or lymphoma (lim-FO-ma), can damage the bone marrow and destroy blood stem cells. Cancer treatments, such as radiation and chemotherapy, also destroy the stem cells.
                
Aplastic Anemia
                
Aplastic anemia is a rare, serious blood disorder in which the bone marrow stops making enough new blood cells. This lowers the number of platelets in your blood.
                
Toxic Chemicals
                
Exposure to toxic chemicalssuch as pesticides, arsenic, and benzenecan slow the production of platelets.
                
Medicines
                
Some medicines, such as diuretics and chloramphenicol, can slow the production of platelets. Chloramphenicol (an antibiotic) rarely is used in the United States.
                
Common over-the-counter medicines, such as aspirin or ibuprofen, also can affect platelets.
                
Alcohol
                
Alcohol also slows the production of platelets. A temporary drop in the platelet count is common among heavy drinkers, especially if they're eating foods that are low in iron, vitamin B12, or folate.
                
Viruses
                
Chickenpox, mumps, rubella, Epstein-Barr virus, or parvovirus can decrease your platelet count for a while. People who have AIDS often develop thrombocytopenia.
                
Genetic Conditions
                
Some genetic conditions can cause low numbers of platelets in the blood. Examples include Wiskott-Aldrich and May-Hegglin syndromes.
                
The Body Destroys Its Own Platelets
                
A low platelet count can occur even if the bone marrow makes enough platelets. The body may destroy its own platelets due to autoimmune diseases, certain medicines, infections, surgery, pregnancy, and some conditions that cause too much blood clotting.
                
Autoimmune Diseases
                
Autoimmune diseases occur if the body's immune system mistakenly attacks healthy cells in the body. If an autoimmune disease destroys the body's platelets, thrombocytopenia can occur.
                
One example of this type of autoimmune disease is immune thrombocytopenia (ITP). ITP is a bleeding disorder in which the blood doesn't clot as it should. An autoimmune response is thought to cause most cases of ITP.
                
Normally, your immune system helps your body fight off infections and diseases. But if you have ITP, your immune system attacks and destroys its own platelets. Why this happens isn't known. (ITP also may occur if the immune system attacks your bone marrow, which makes platelets.)
                
Other autoimmune diseases that destroy platelets include lupus and rheumatoid arthritis.
                
Medicines
                
A reaction to medicine can confuse your body and cause it to destroy its platelets. Examples of medicines that may cause this to happen include quinine; antibiotics that contain sulfa; and some medicines for seizures, such as Dilantin, vancomycin, and rifampin. (Quinine is a substance often found in tonic water and nutritional health products.)
                
Heparin is a medicine commonly used to prevent blood clots. But an immune reaction may trigger the medicine to cause blood clots and thrombocytopenia. This condition is called heparin-induced thrombocytopenia (HIT). HIT rarely occurs outside of a hospital.
                
In HIT, the body's immune system attacks a substance formed by heparin and a protein on the surface of the platelets. This attack activates the platelets and they start to form blood clots.
                
Blood clots can form deep in the legs (deep vein thrombosis), or they can break loose and travel to the lungs (pulmonary embolism).
                
Infection
                
A low platelet count can occur after blood poisoning from a widespread bacterial infection. A virus, such as mononucleosis or cytomegalovirus, also can cause a low platelet count.
                
Surgery
                
Platelets can be destroyed when they pass through man-made heart valves, blood vessel grafts, or machines and tubing used for blood transfusions or bypass surgery.
                
Pregnancy
                
About 5 percent of pregnant women develop mild thrombocytopenia when they're close to delivery. The exact cause isn't known for sure.
                
Rare and Serious Conditions That Cause Blood Clots
                
Some rare and serious conditions can cause a low platelet count. Two examples are thrombotic thrombocytopenic purpura (TTP) and disseminated intravascular coagulation (DIC).
                
TTP is a rare blood condition. It causes blood clots to form in the body's small blood vessels, including vessels in the brains, kidneys, and heart.
                
DIC is a rare complication of pregnancy, severe infections, or severe trauma. Tiny blood clots form suddenly throughout the body.
                
In both conditions, the blood clots use up many of the blood's platelets.
                
The Spleen Holds On to Too Many Platelets
                
Usually, one-third of the body's platelets are held in the spleen. If the spleen is enlarged, it will hold on to too many platelets. This means that not enough platelets will circulate in the blood.
                
An enlarged spleen often is due to cancer or severe liver disease, such as cirrhosis (sir-RO-sis). Cirrhosis is a disease in which the liver is scarred. This prevents it from working well.
                
An enlarged spleen also might be due to a bone marrow condition, such as myelofibrosis (MI-eh-lo-fi-BRO-sis). With this condition, the bone marrow is scarred and isn't able to make blood cells.