In the body l-phenylalanine is converted to l-tyrosine. The amino acid l-tyrosine is a precursor of catecholamine neurotransmitters, including l-dopa, dopamine, epinephrine and norepinephrine. These neurotransmitters are responsible for supporting emotional well-being and mental function. l- Tyrosine plays a role in promoting healthy neurotransmitter function in response to environmental and emotional stress. Research has indicated its potential to further support cognitive function and memory under stressful conditions. In one study, l-tyrosine helped support mood in subjects with low dopamine levels. l-Tyrosine is also a key component in the synthesis of thyroid hormones.