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Dietary SupplementFree FormAmino AcidL-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.