Alternative Medicine Review 2007 (Dec): 12 (4): 364–368 ~ FULL TEXT
L-tyrosine is a conditionally essential amino acid because under normal conditions the body synthesizes sufficient quantities from phenylalanine.  For those with phenylketonuria, however, a severe deficiency in the enzyme phenylalanine hydroxylase prevents conversion of phenylalanine to tyrosine, making tyrosine an essential amino acid for this population.  Tyrosine is incorporated into proteins of all life forms and is a precursor for synthesis of thyroxin, melanin, and the neurotransmitters dopamine and norepinephrine. [1, 2] Food sources of tyrosine include fish, soy products, poultry, eggs, dairy products, lima beans, almonds, peanuts, sesame seeds, pumpkin seeds, wheat germ, oats, avocados, and bananas. [1, 3] Clinical conditions for which tyrosine supplementation may be of therapeutic benefit include depression, hypertension, stress, cognitive function and memory, Parkinson’s disease, phenylketonuria, and narcolepsy.