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.
Absorption pharmacokinetics of a single oral dose of L-tyrosine was studied in 12 normal fasting subjects
(ages 18-21). Six subjects in each group fasted overnight then were given either 100 or 150 mg/kg tyrosine and continued to fast for another eight hours. Peak plasma tyrosine levels were attained at two hours post-ingestion and remained elevated above baseline for 6-8 hours. For those taking 100 mg/kg tyrosine, plasma levels rose from 69 nmols/mL (±3.9) to 154 nmols/mL (±9.5); for those receiving 150 mg/kg tyrosine, plasma levels rose to 203±31.5 nmols/mL. No side effects were noted. The flow of tyrosine across the blood-brain barrier and brain tyrosine levels are dependent on the ratio of plasma tyrosine to the total plasma concentrations of other large neutral amino acids (phenylalanine, tryptophan, methionine, valine, leucine, and isoleucine) that compete for neuronal uptake in the brain.  Animal studies demonstrate brain tyrosine levels enhance neurotransmitter synthesis, [5, 6] and research in humans seems to indicate the same. 
L-tyrosine is absorbed from the small intestine and transported to the liver via the portal circulation. L-tyrosine not utilized by the liver enters the systemic circulation and is distributed to various body tissues1
where it is utilized in three different metabolic pathways:
(1) absorbed into the tissues and incorporated into proteins and peptides;
(2) used as precursors in smaller amounts for thyroxin, melanin, and neurotransmitter synthesis; or
(3) deaminated to form the gluconeogenesis substrate, p-hydroxy phenylpyruvic acid.
In the latter process, the enzyme catalyzing this reaction (tyrosine transaminase) exhibits a marked diurnal variation in liver concentrations, causing a similar degree of diurnal variation in plasma tyrosine levels in normal humans.