L-tyrosine is an amino acid precursor for synthesis of the neurotransmitters norepinephrine and dopamine. By improving the rate of It appears to function as an adaptogen by relieving physical symptoms of stress, such as mood swings. L-tyrosine also serves as a precursor for the thyroid hormone thyroxine and melanin, the pigment responsible for skin and hair color and protection against harmful ultraviolet rays. L-tyrosine is also a powerful antioxidant, scavenging and neutralizing free radicals and inhibiting fat oxidation.