Although the causes of depression cannot be fully understood, they are extremely vast and varied. Depression can be triggered by a number of things including: chemical imbalances in the brain, tension, stress, thyroid disorder, a traumatic life event, headaches, poor diet, mononucleosis, lack of exercise, nutritional deficiencies, the consumption of sugar, endometriosis, and hormone imbalance which is caused by hormone replacement therapy and oral contraceptives.

Symptoms of depression last anywhere from weeks, to months, to years. It differs with each  individual. There also are different types of this disease, with variations occurring in the amount of symptoms, their persistence and severity.

Regardless of the factors that seem to trigger depression, it initially starts with a disturbance in the part of the brain that controls our moods. Most of our bodies have the ability to readjust to pressures, so most of us have can endure a fair amount of stress. If the stress gets too great, and the adjustment mechanism in our brain is unresponsive, depression could be triggered. 

Progesterone and Depression
When our hormones are out of balance, our whole body is affected. Frequently, a woman on hormone replacement therapy is estrogen dominant and progesterone deficient. When our progesterone levels are in balance, excess estrogen is managed. As stated below, the effects of estrogen in comparison to those of progesterone differ when dealing with depression.