Hot flashes are mostly caused by the hormonal changes of menopause. As your estrogen level falls, this has a direct effect on the hypothalamus, the part of the brain responsible for controlling your appetite, sleep cycles, sex hormones, and body temperature. Somehow (we don’t know exactly how), the drop in estrogen confuses the hypothalamus — which is sometimes referred to as the body’s “thermostat”— and makes it read “too hot.”
The brain responds to this report by broadcasting an all-out alert to the heart, blood vessels, and nervous system: “Get rid of the heat!” The message is transmitted by the nervous system's chemical messenger, epinephrine, and related compounds: norepinephrine, prostaglandin, and serotonin. The message is delivered instantly. Your heart pumps faster, the blood vessels in your skin dilate to circulate more blood to radiate off the heat, and your sweat glands release sweat to cool you off even more.
This heat-releasing mechanism is how your body keeps you from overheating in the summer, but when the process is triggered instead by a drop in estrogen, your brain's confused response can make you very uncomfortable. Some women's skin temperature can rise six degrees Centigrade during a hot flash. Your body works to cool itself down when it shouldn’t, and you are miserable: soaking wet in the middle of a work meeting or in the middle of a good night’s sleep.
Hormonal therapies that lower estrogen levels or block the hormone’s effects also can trigger this process. For example, tamoxifen causes hot flashes in more than 50% of users. The hot flashes usually increase over the first several months of treatment and then get better. If you’re already postmenopausal and had hot flashes before using tamoxifen, you’re likely to experience more severe hot flashes with this medication.