Tetrahydrobiopterin (BH4, THB), also known as sapropterin (INN), is a cofactor of the three aromatic amino acid hydroxylase enzymes, used in the degradation of amino acid phenylalanine and in the biosynthesis of the neurotransmitters serotonin (5-hydroxytryptamine, 5-HT), melatonin, dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline), and is a cofactor for the production of nitric oxide (NO) by the nitric oxide syntheses.Chemically, its structure is that of a (dihydropteridine reductase) reduced pteridine derivative (Quinonoid dihydrobiopterin).

Tetrahydrobiopterin is available as a tablet for oral administration in the form of sapropterin dihydrochloride (BH4*2HCL). It was approved for use in the United States as a tablet in December 2007 and as a powder in December 2013. It was approved for use in the European Union in December 2008, Canada in April 2010, and Japan in July 2008. It is sold under the brand names Kuvan and Biopten. The typical cost of treating a patient with Kuvan is US$100,000 per year. BioMarin holds the patent for Kuvan until at least 2024, but Par Pharmaceutical has a right to produce a generic version by 2020.
Sapropterin is indicated in tetrahydrobiopterin deficiency caused by GTP cyclohydrolase I (GTPCH) deficiency, or 6-pyruvoyltetrahydropterin synthase (PTPS) deficiency. Also, BH4*2HCL is FDA approved for use in phenylketonuria (PKU), along with dietary measures.