L-Tyrosine is an amino acid found at relatively high levels in the average diet, and is tasked with helping to produce hormones such as adrenaline and dopamine. It is primarily included in weight loss and pre-workout products as a way of increasing perceived energy without upping the total stimulant content of a product.
Tyrosine provides its beneficial actions due to catecholamine synthesis, in that it helps the body make additional adrenaline and dopamine. Another proposed mechanism of tyrosine is its ability to prolong the beneficial effects of cathecholamines such as noradrenaline, allowing the body to work better under stress.
Tyrosine is commonly dosed in the 500mg-1000mg range, although some athletes and studies have shown benefits with up to 2000mg/day.
In some extreme studies, doses of 9-13.5g for a 200lbs athlete have been studied, however this style of high dose is not recommended.