Tyrosine is an essential amino acid that has the ability to cross the blood-brain barrier and is a precursor to the neurotransmitters dopamine, norepinephrine and epinephrine (better known as adrenaline) which play an important role in the functioning of the sympathetic nervous system. Tyrosine is a precursor to thyroxine, catecholamines and the most important human pigment, melanin, it plays an important role in the functioning of the adrenal glands, pituitary gland and thyroid gland while it is not found in large quantities in the body because it is metabolized at a rapid rate.