Amino Acid Supplement L-Tyrosine is a free-form amino acid which rapidly crosses the blood-brain barrier. It is a precursor for the neurotransmitters dopamine, norepinephrine and epinephrine, as well as for thyroid hormones and enkephalins, special peptides which subdue messages of physical discomfort. Tyrosine is also a constituent of amino sugars (glycosaminoglycans), special compounds which have important roles throughout the body.