Here is something I had found out about from a conversation I had with a female naturopathic doctor that most MD's specializing in transgender medications either don't consider or fail to discuss, probably because MtF's are usually past puberty when starting transition and will never get pregnant. It concerns prolactin and it's effects on breast development. Here is a short article that corresponds to that conversation.
"Your body generally only secretes prolactin at two specific times: during puberty and during pregnancy. Because it plays such a key role in breast development, though, prolactin is one hormone that you definitely want to get to know. During puberty, prolactin works with estrogen to develop the mammary glands, while increasing the number of estrogen receptors in the breasts. Prolactin also causes the breasts to store fat by increasing the production of lipoprotein lipase (LPL) in your breasts.5 LPL is a "fat storage" enzyme. During pregnancy and while nursing, prolactin controls the production of milk. It is first secreted around the eighth week of pregnancy and peaks at birth. After that, prolactin is only produced according to stimulation - namely, a baby suckling at your nipple. Interestingly enough, you don't actually have to be pregnant to produce prolactin. Adequate prolactin levels are absolutely crucial for triggering breast growth.
Growth hormone, as the name implies, triggers the growth of your body. It also plays a special role in breast development. Growth hormone is so important that it is one of your body's most plentiful hormones. It is produced by the pituitary gland during the first two hours of sleep. Almost immediately after it's secreted, growth hormone is converted in the liver into a new, more usable substance known as Insulin like Growth Factor (IGF). Growth hormone could be thought of as your body's "Master Hormone" since it regulates every other hormone you've got. Growth hormone is produced in massive amounts during puberty, promoting the growth of cells, tissues, and organs. Without enough growth hormone, your body and your breasts simply wouldn't grow. As an adult, growth hormone is involved in tissue repair and cell replacement. Unfortunately, growth hormone levels start to plummet after the age of thirty. By the time you hit sixty, your growth hormone level could be 75% lower than it was when you were twenty! Low growth hormone levels are directly responsible for the aging of the body. Breast enlargement requires a lot of growth hormone. You had a ton of it at your disposal during puberty, but if you're over the age of eighteen, you're going to have to boost it back up in order to get your breasts to grow again."
