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.