As far as Hormone Replacement Therapy is concerned, thrombosis refers to deep vein thrombosis. These are blood clots that can form that can partially or completely block a blood vessel. The condition occurs most frequently in the legs but it can occur almost anywhere from the lower abdomen down. On average, deep vein thrombosis is fairly rare and affects about 1 person in 1700.
People who are particularly susceptible are the elderly, those that are overweight and those who have a condition called polycythaemia (inability to control the rise in numbers of red blood cells). Deep vein thrombosis can cause pulmonary embolism which is potentially fatal. A pulmonary embolism happens when pieces of a blood clot break away and becomes lodged in an artery in the lungs thus reducing the amount of oxygenated blood going to the heart.
The incidence of deep vein thrombosis appears to be increased two – four folds in HRT users. According to John Studd, the risk seems to be higher when treatment is started and reduces significantly in long-term users.
In a woman that is categorised as low risk, this may mean an increase of 1:5000. Any woman that already has a history of deep vein thrombosis or pulmonary embolism should avoid taking HRT unless the thrombosis was as a result of surgery, accident, pregnancy or childbirth in which case there should not be an increased risk.