The alpha chain of pituitary Glycoprotein Hormones (Thyrotropin; Follicle Stimulating Hormone; Luteinizing Hormone) and the placental Chorionic Gonadotropin. Within a species, the alpha subunits of these four Hormones are identical; the distinct functional characteristics of these Glycoprotein Hormones are determined by the unique beta subunits. Both subunits, the non-covalently bound heterodimers, are required for full biologic activity.