TH and phenylketonuria: BH4 supplementation therapy (sapropterin dihydrochloride, Kuvan®) is used for BH4 deficiencies [79] and PAH deficiency [80]. For TH deficiencies, BH4 has demonstrated to stabilize TH in vitro and in vivo, increasing TH levels and activity [81] and improves phenotype in human iPSCs and in the THD mouse model [82].