Premenstrual Dysphoric Disorder (PMDD) is a severe form of premenstrual syndrome (PMS) that causes physical and emotional symptoms every menstrual cycle in the week or two before your period. The exact cause of PMDD is not known, but it may be an abnormal reaction to normal hormone changes that happen with each menstrual cycle. The hormone changes can cause a serotonin deficiency, which is a substance found naturally in the brain and intestines that narrows blood vessels and can affect mood and cause physical symptoms. Serotonin levels, like hormone levels, change throughout the menstrual cycle, and some women may be more prone to PMDD if they have an increased sensitivity to their reproductive hormones during the two weeks before their period starts. Other possible causes and factors that may make PMDD worse include genetics, smoking, and inflammation. It is important to note that the symptoms of PMDD may look like other conditions or medical problems, such as a thyroid condition, depression, or an anxiety disorder, so it is essential to talk with a healthcare provider for a diagnosis.