Tyrosine is an essential amino acid which can go. Through the blood-brain barrier
Once in the brains is a precursor of the neurotransmitters dopamine, norepinephrine and epinephrine.
These neurotransmitters are an important part of the sympathetic nervous system. Tyrosine is the precursor of several hormones, including thyroid hormone and catecholamines, and the major human pigment, melanin. It also plays a role in the functioning of the adrenal, pituitary and thyroid.
Tyrosine is not found in large quantities in the body because it is rapidly converted.