Around age 45 to 50, some times a little earlier or later, estrogen levels begin to fall. When they fall below the levels necessary to signal the uterine lining to thicken and gather blood, the menstrual flow becomes less and/or irregular, eventually stopping altogether. 

Let's zero in and take a closer look at Hot Flashes, the hallmark of menopausal symptoms. The prevailing explanation for hot flashes is as follows: recall that an area (which we'll call the GnRH center) in the brain's hypothalamus monitors estrogen and progesterone levels. When levels fall, this center makes GnRH which stimulates the pituitary to make hormones, (FSH and LH) , which in turn result in the ovarian production of estrogen and progesterone. The rise in these hormones inhibits further production of GnRH. At menopause, estrogen levels fall and progesterone levels are usually already low. The ovaries no longer respond to the FSH and LH prompt. 

In addition to hot flashes, the heightened activity of the hypothalamus can cause mood swings, fatigue, feelings of being cold, and inappropriate responses to other stressors. Many women will have symptoms of hypothyroidism despite normal thyroid hormone levels. 

In a nutshell:

• The GnRH center effectively signals to increase estrogen and progesterone levels. 
• Elevated estrogen and progesterone inhibit GnRH release. 
• After menopause the ovaries no longer make estrogen and progesterone. 
• Lack of estrogen and progesterone response results in increased activity of the GnRH center. 
• Heightened GnRH activity activates the vasomotor center, causing hot flashes and perspiration.