T4 is the other major hormone produced by the thyroid, and it is particularly important to maintaining a healthy metabolism, mood, and a stable body temperature. When T3 levels get low, T4 can also be converted into T3 for use throughout the body. Too much T4 can disrupt your menstrual cycle and cause your hair to thin, among other issues, while too little T4 can cause infertility. T4 plays a key role in promoting wound healing on the skin along with HGH and insulin.