What is L-Tyrosine?
LTyrosine is an amino acid found in the diet that is involved in producing catecholamines such as dopamine and adrenaline, specifically it is the amino acid that itself is metabolized to produce these neurotransmitters. It is investigated as a supplement based on the idea that supplementing L-tyrosine will provide more 'building blocks' for these catecholamines to be produced and hopefully, indirectly, provide benefits secondary to them. As it is also structurally related to thyroid hormones meaning that, due to relations to both adrenaline and the thyroid, it is commonly found in fat loss supplements to a degree.
When it comes to the topic of actually increasing dopamine and adrenaline outright, L-tyrosine does not seem to hold much promise. The synthesis of catecholamines is highly regulated in the body and in particular the one enzyme that converts L-tyrosine into the next metabolite, L-DOPA, so simply increasing the amount of L-tyrosine in your body does not necessarily increase catecholamine production.
However some studies have noted that in stressful situations where the catecholamine known as noradrenaline exerts a protective effect during the stress, supplying some L-tyrosine seems to provide a 'safety buffer' of building blocks that can prolong the anti-stress effect of catecholamines by delaying their depletion. This has been noted in situations of cold stress and sleep deprivation in humans, but at a relatively high dose of 150mg.
How to Take?
Anecdotally, L-Tyrosine tends to be taken in doses of 500-2000mg approximately 30-60 minutes before any acute stressor (this tends to be exercise)
Studies in humans showing most anti-stress promise for acute supplemental L-Tyrosine use a dosage range of 100-150mg/kg bodyweight which can be taken 60 minutes before exercise; this is a dosage range of 9-13.5g for a 200lb person and 7-10g for a 150lb person.