The beta subunit of luteinizing hormone. It is a 15-kDa glycopolypeptide with structure similar to the beta subunit of the placental chorionic gonadatropin (CHORIONIC GONADOTROPIN, BETA SUBUNIT, HUMAN) except for the additional 31 amino acids at the C-terminal of CG-beta. Full biological activity of LH requires the non-covalently bound heterodimers of an alpha and a beta subunit. Mutation of the LHB gene causes HYPOGONADISM and infertility.