Science-Based Nutrition Dietary Supplement Brain Transmitter Support Non-GMO Gluten Free Tyrosine is conditionally essential amino acid that is a precursor to the catecholamine transmitters dopamine, epinephrine, and norepinephrine. These are vital messengers for the brain and the body's other nerve networks. The thyroid gland also needs tyrosine to make the vital hormone thyroxine. Precursor for the nerve messengers dopamine, norepinephrine, and epinephrine (adrenaline) Precursor for thyroxine production by the thyroid gland