THNSL1 (threonine synthase-like 1), also known as TSH1, is a 743 amino acid member of the serine/threonine dehydratase family. Expressed primarily in brain and endocrine glands, THNSL1 is thought to function as a pyridoxal-5'-phosphate (PLP)-dependent enzyme that uses pyridoxal phosphate as a cofactor. THNSL1 shares similarity with bacterial threonine synthases (which synthesize threonine from aspartic acid), suggesting that THNSL1 may have once participated in de novo threonine synthesis within the body, but has since lost its original metabolic role.