We usually think of the word synthetic as meaning something that is produced artificially, and is not found in nature, such as plastics and pharmaceutical drugs. For example, the "hormone" Provera is made from the same substances that natural progesterone is made from, but the molecular configuration of it is changed in the laboratory so that it is not identical to anything found in nature. But natural progesterone made in the laboratory is identical to that made in the human body. In other words, what makes a substance "synthetic" or "natural" in this context is whether or not it can be found in nature.

The same distinctions apply to estrogens. The two types of estrogen in Premarin taken separately are natural (found in nature) and not synthetic, but not all of the estrogen in Premarin is natural to humans. About half of it is human estrogen, and about half is horse estrogen -- a molecule not found in the human body. It's ironic that the manufacturer of Premarin has tried to advertise it as a natural product. Since about half of the estrogen in Premarin is estrone (which is natural to humans) and the other half consists of a different estrogen that is natural only to horses and is extracted from pregnant horse urine, it is natural, strictly speaking, only if you are half horse and half human! It's unfortunate that so much of estrogen research has been done with Premarin, so we don't have a truly accurate knowledge base of the effects of human estrogen versus horse estrogen.

Natural estrogens extracted from wild yams or soybeans that are identical to those made by the human body are easily available by prescription in the form of creams, tablets and patches. These are estrone, estradiol and estriol, so there is no reason to take horse estrogen.

Plants do not make human hormones, but some plants make compounds that have some hormonal effect. These, in their natural form, are called phytohormones ("plant-based" hormones). Although they are not the same as our hormones they may have some hormonal activity. 

Some plants make substances that are quite similar to animal cholesterol or animal hormones, but they have no hormonal effect. Such compounds, called sterols, are easily chemically modified so that they become identical to human hormones. An example of this is the diosgenin that is extracted from wild yams and soybeans to make human hormones in the laboratory.