Ever since my own experience with recurrent miscarriages, I've continued to be interested in all the theoretical causes of the condition being researched -- especially the causes related to hormonal imbalances. Hormonal issues have not been proven as a cause of recurrent miscarriages, but researchers have looked at numerous different hormonal factors as possibly playing a role.
Most people who have multiple miscarriages come across the idea of progesterone supplementation, a controversial but commonly suggested therapy for preventing miscarriages (progesterone has never been proven to reduce risk of miscarriage). Fewer people have heard of prolactin, another important hormone in the body. A few studies have correlated high prolactin levels (hyperprolactinemia) to risk of recurrent miscarriages, and some doctors even test for and treat high prolactin levels in women with recurrent miscarriages, but currently there's no definitive proof behind this theory. It is, however, an interesting area of research.