l-Tyrosine is a free-form amino acid which crosses the blood-brain barrier. It is a precursor for the motivational neurotransmitters dopamine, norepinephrine, and epinephrine, as well as for thyroid hormones and enkephalins, important peptides which ease messages of physical discomfort. l-Tyrosine is also a constituent of amino sugars (glycosaminoglycans), which play a role in numerous biological functions.*