Pure DL-Phenylalanine 90 capsulesPrecursor to various messenger substances Phenylalanine is an essential amino acid that the body cannot produce itself. The substance, which occurs naturally as a protein component, must therefore be supplied through the diet. Phenylalanine is converted to tyrosine in the liver. This amino acid is an important starting substance for the thyroid hormones. The messenger substances adrenaline, noradrenaline and dopamine are also formed from tyrosine.