Polarized light micrograph of tyrosine crystals, a non-essential amino acid used by cells to synthesize proteins. Tyrosine phosphorylation is a key step in signal transduction, enzymatic activity and other intracellular processes. Tyrosine readily passes the blood-brain barrier, and in the brain it is a precursor for the neurotransmitters dopamine, norepinephrine, and epinephrine. The thyroid hormones triiodothyronine and thyroxine are derived from tyrosine. Tyrosine can help combat depression, stress, narcolepsy, and chronic fatigue, as well as to improve memory. It is used for the treatment of hyperthyroidism, polio, encephalitis, and Parkinson's. It is also related to schizophrenia, epilepsy, Alzheimer's disease, cachexia, hawkinsinuria, alkaptonuria, leukemia, hypothyroidism, myocardial infarction, and albinism. Field of view size: 0.30 x 0.45 mm (0.012 x 0.018 in)