Phenylalanine is an essential amino acid that the body cannot produce itself. Therefore, this naturally occurring protein component must be obtained from the diet. Phenylalanine is converted to tyrosine in the liver. This amino acid is an important precursor to thyroid hormones. The substances adrenaline, noradrenaline, and dopamine are also formed from tyrosine. Pure Encapsulations DL-phenylalanine provides a balanced ratio of the D- and L-forms.