pure encapsulations l-tyrosine | amino acid supplement for sleep, thyroid support, cognitive health, and memory* | 90 capsules pure encapsulations 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 well-being and mental function. l-tyrosine plays a role in promoting healthy neurotransmitter function in response to stress. research has indicated its potential to further support cognitive function and memory under stressful conditions. in one study, l-tyrosine helped support sleep in subjects with low dopamine levels. l-tyrosine is also a key component in the synthesis of thyroid hormones.