*In the body, phenylalanine is converted to tyrosine. The amino acid 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 promote cognitive function and memory under stressful conditions. In one study, l-tyrosine enhanced mood and sleep in subjects with low dopamine levels. Tyrosine is also a key component in the synthesis of thyroid hormones.