L-Tyrosine is a naturally occurring form of tyrosine, an amino acid the body uses to build proteins, neurotransmitters, and other vital compounds. (1) It is metabolized to produce catecholamines such as dopamine and adrenaline. Tyrosine is essential to your body's production of melanin (a type of pigment) and several key brain chemicals including dopamine and norepinephrine. It also plays an important role in the function of the adrenal, thyroid, and pituitary glands, which are involved in producing and regulating hormones. Many bodybuilders choose to supplement with L-Tyrosine to help stimulate HGH production by the pituitary glands.

Some studies show the use of tyrosine supplements may boost brain function under certain conditions. A 2010 study published in the Journal of the International Society of Sports Nutrition found that tyrosine supplementation led to significantly greater improvements in focus and alertness after exhaustive exercise. (2) The study included 19 healthy college students, each of whom was given either a tyrosine supplement or a placebo for a four-week period. In addition, a 2007 study of 19 people published in Physiology & Behavior found that the use of tyrosine supplements helped protect against the detrimental effects of severe cold exposure on cognitive performance and memory. (3) Studies show that Tyrosine can help maintain mental capacity when taken before a stressful activity. Additionally, supplementing with tyrosine can reverse mental decline and improve cognition in short-term stressful or mentally demanding situations. (4) 

Tyrosine supplement also helps prevent degradation of performance due to loss of sleep. A single dose of it helped people who lost a night’s sleep stay alert for three hours longer than they otherwise would. (4) Low levels of tyrosine have also been linked to autism. (5) There was a trend for children with autism who were on restricted diets (gluten and casein restricted diets administered by parents) to have an increased prevalence of essential amino acid deficiencies and lower plasma levels of essential acids including the neurotransmitter precursors tyrosine and tryptophan than both controls and children with autism on unrestricted diets.

According to a study of 10 men and women, administration of 150 mg/kg of L-Tyrosine significantly enhanced accuracy and decreased frequency of list retrieval on the working memory task during the multiple task battery compared with placebo. (6) Also, there is evidence that L-Tyrosine helps prevent depression. Since tyrosine enables the production of dopamine and noradrenaline, it may act as a nutritional mood support. (7) L-Tyrosine also helps balance thyroid hormones. In 85 volunteers, supplementation with high doses of L-Tyrosine (12g daily) during harsh winter slightly increased T3, the active thyroid hormone. (8) It also greatly reduced TSH, high levels of which are linked with hypothyroidism and stress. Tyrosine supplementation prevented thyroid hormone drops in mice exposed to chronic stress (9)

References:

1. Tyrosine National Center for Biotechnology Information. PubChem Database. Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/Tyrosine

2. Hoffman JR, Ratamess NA, Gonzalez A, Beller NA, Hoffman MW, Olson M, Purpura M, Jäger R. The effects of acute and prolonged CRAM supplementation on reaction time and subjective measures of focus and alertness in healthy college students. J Int Soc Sports Nutr. 2010 Dec 15; 7-39.

3. Mahoney CR, Castellani J, Kramer FM, Young A, Lieberman HR. Tyrosine supplementation mitigates working memory decrements during cold exposure. Physiol Behav. 2007 Nov 23; 92 (4): 575-82.

4. Neri DF, Wiegmann D, Stanny RR, Shappell SA, McCardie A, McKay DL. The effects of tyrosine on cognitive performance during extended wakefulness. Aviat Space Environ Med. 1995 Apr; 66 (4): 313-9.

5. Arnold GL, Hyman SL, Mooney RA, Kirby RS. Plasma amino acids profiles in children with autism: potential risk of nutritional deficiencies. J Autism Dev Disord. 2003 Aug; 33 (4): 449-54.

6. Thomas JR, Lockwood PA, Singh A, Deuster PA. Tyrosine improves working memory in a multitasking environment. Pharmacol Biochem Behav. 1999 Nov; 64 (3): 495-500.

7. Gelenberg AJ, Wojcik JD, Gibson CJ, Wurtman RJ. Tyrosine for depression. J Psychiatr Res. 1982-1983; 17 (2): 175-80.

8. Palinkas LA, Reedy KR, Smith M, Anghel M, Steel GD, Reeves D, Shurtleff D, Case HS, Van Do N, Reed HL. Psychoneuroendocrine effects of combined thyroxine and triiodothyronine versus tyrosine during prolonged Antarctic residence. Int J Circumpolar Health. 2007 Dec; 66 (5): 401-17.

9. Zhihua Wang, Jinghua Li, Zhiming Wang, Lingyan Xue, Yi Zhang, Yingjie Chen, Jun Su, Zhongming Li. L-tyrosine improves neuroendocrine function in a mouse model of chronic stress Neural Regen Res. 2012 Jun 25; 7 (18): 1413–1419.