Phenylalanine What Is Phenylalanine? Phenylalanine is an essential amino acid (because the body cannot produce sufficient quantities) that is required for normal protein synthesis. Phenylalanine is converted to tyrosine in the body. It is a precursor to the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline), as well as thyroid hormones and melanin (a naturally occurring pigment in the skin that gives it colour). https://www.nationalnutrition.ca/amino-acids/single-aminos/phenylalanine.html #aminoacids