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L-tyrosine is a nonessential amino acid (protein building block) that the body synthesizes from phenylalanine, another amino acid. Tyrosine is important to the structure of almost all proteins in the body. It is also the precursor of several neurotransmitters, including L-dopa, dopamine, norepinephrine, and epinephrine. L-tyrosine, through its effect on neurotransmitters, may affect several health conditions, including Parkinson's disease, depression, and other mood disorders. Studies have suggested that tyrosine may help people with depression. Preliminary findings indicate a beneficial effect of tyrosine, along with other amino acids, in people affected by dementia, including Alzheimer's disease. Due to its role as a precursor to norepinephrine and epinephrine (two of the body's main stress-related hormones) tyrosine may also ease the adverse effects of environmental, psychosocial, and physical stress.

L-tyrosine is converted by skin cells into melanin, the dark pigment that protects against the harmful effects of ultraviolet light. Thyroid hormones, which have a role in almost every process in the body, also contain tyrosine as part of their structure.