What exact kind of hormonal imbalance causes menorrhagia?

Thyroid/Pituitary. Menorrhagia/Heavy Menstrual Bleeding is any period bleeding that leads to soaking a pad/tampon more than every 2hrs, interferes w/ daily activities, or lasts >7days. There are many causes: fibroids, polyp, hormonal imbalance (TSH/Prog/Estr), adenomyosis, bleeding disorders and cancer. Get checked out. Taking progesterone (pills/IUD) can help to thin the uterine lining so you don't bleed as much.
Could be . Almost any but if you have irregular periods too most likely is an imbalance of fsh and lh hormones.