Estrogen is not a hormone; it's a class of hormones.

Almost every living thing makes some form of estrogen, including plants and animals. But the estrogens found in plants are quite different than the estrogens found in animals. And each species of animal has his own unique mixture of estrogens. Human beings make a unique mixture of estrogens not produced by any other animal on the planet.

But traditionally, when women go to the doctor for treatment of their menopausal symptoms 95% of the time they are given a compound known as conjugated equine estrogens. These are estrogens extracted from the urine of pregnant horses, and represent a mixture of estrogens that are quite different from those normally produced by human beings.

It's always struck me as odd that the standard of care is to re-create the normal hormone balance of a pregnant horse in a non-pregnant human being.