Who should not take L-tyrosine

L-tyrosine is often presented as an "ordinary amino acid from food" that will harm no one. For most healthy adults it does indeed have a favorable safety profile. However, there are groups of people for whom additional tyrosine in doses that significantly exceed dietary ones may be dangerous or at least inadvisable. The editors have gathered these cases and explained where the risk comes from.

Why an amino acid can have contraindications

Tyrosine is not an inert substance but the starting material for several biologically active compounds: dopamine, noradrenaline, adrenaline, thyroxine, triiodothyronine and melanin. In food it arrives together with other amino acids, gradually and in moderate amounts. A supplement, however, creates a short-term "spike" in concentration, which in certain conditions the body handles worse.

The second reason is transport. Tyrosine enters the brain via the same large neutral amino acid transporter (LAT1) used by tryptophan, leucine, phenylalanine and some medications. A large dose of tyrosine can change the balance in this "queue," which matters for people taking such drugs.

The third is metabolism. Tyrosine is broken down in the liver by a chain of enzymes. If one of them does not work because of a genetic defect, intermediate products accumulate and become toxic. For such people even dietary tyrosine is limited, and a supplement is categorically contraindicated.

Finally, the studies underlying tyrosine’s popularity were conducted mostly on young healthy volunteers, military personnel and students. Data on safety in pregnant women, children, the elderly and patients with chronic diseases are limited, so for these groups the precautionary principle applies.

Hereditary disorders of tyrosine metabolism

The strictest contraindications are associated with rare genetic diseases. Type I tyrosinemia arises from a deficiency of the enzyme fumarylacetoacetate hydrolase; the accumulation of toxic metabolites damages the liver and kidneys. Treatment includes the drug nitisinone and a diet restricting tyrosine and phenylalanine (Chinsky et al., 2017). Tyrosine supplementation is unacceptable for such patients.

Type II and III tyrosinemias, as well as alkaptonuria, are other defects of the same breakdown pathway. They manifest as damage to the eyes and skin, intellectual impairment, or damage to the joints and connective tissue. What they all share: controlling tyrosine intake is part of treatment, not a reason for "additional support."

A separate case is phenylketonuria (PKU). Here phenylalanine hydroxylase does not work, so the body cannot synthesize tyrosine from phenylalanine, and tyrosine becomes an essential amino acid for the patient. Medical formulas for people with PKU already contain tyrosine in a calculated amount (van Spronsen et al., 2021).

Therefore, for people with PKU tyrosine is not forbidden, but any additional intake must be coordinated with a metabolic center. Self-directed supplements can upset the balance that the doctor maintains through regular blood tests.

The thyroid gland and hormone therapy

The thyroid hormones — thyroxine (T4) and triiodothyronine (T3) — are formed by iodination of tyrosine residues in the protein thyroglobulin. On this basis tyrosine is often advertised as "thyroid support." In reality, in a healthy person hormone synthesis is limited not by tyrosine but by iodine and regulation from the pituitary gland.

Nevertheless, for people with hyperthyroidism, diffuse toxic goiter (Graves' disease) or toxic nodular goiter, the editors do not recommend taking tyrosine without an endocrinologist's consent. Although direct evidence of harm is limited, the condition itself is already characterized by an excess of hormones, and tyrosine additionally boosts catecholamine synthesis, so palpitations and anxiety may intensify.

For people taking levothyroxine for hypothyroidism, tyrosine will not replace treatment and will not make it "more natural." The hormone dose is selected by TSH level; "thyroid" complexes with tyrosine, iodine and animal thyroid tissue can distort test results and complicate the selection of therapy.

If a person still wants to take tyrosine for other reasons, they should inform their endocrinologist and, if necessary, check TSH after starting. This is a simple measure that allows an unwanted change to be noticed in time.

Interaction with medications

The most serious pharmacological caution concerns monoamine oxidase inhibitors (MAOIs) — an old class of antidepressants, as well as selegiline and rasagiline used in Parkinson's disease. MAO breaks down catecholamines; if it is blocked, an increase in their synthesis could theoretically contribute to a sharp rise in blood pressure. Although classic hypertensive crises on MAOIs are associated with tyramine rather than tyrosine, the combination is considered undesirable.

Levodopa — the main drug for treating Parkinson's disease — is absorbed in the intestine and enters the brain via the same large neutral amino acid transporter. The classic work by Nutt and colleagues (1984) showed that a high-protein meal and amino acids in the blood weaken the effect of levodopa. Large doses of tyrosine can create similar competition.

Tyrosine and stimulants should be combined with caution: ADHD medications, decongestants with pseudoephedrine, "fat burners." Tyrosine itself is not a stimulant, but in combination with substances that release catecholamines it can increase insomnia, anxiety and tachycardia.

Finally, thyroid hormones and antithyroid drugs (thiamazole, propylthiouracil) are another group where a doctor's consultation is needed, as discussed above.

• MAO inhibitors, including selegiline and rasagiline — the combination is undesirable.
• Levodopa — possible competition for absorption and transport into the brain.
• Levothyroxine, thiamazole, propylthiouracil — only with an endocrinologist's knowledge.
• Stimulants and decongestants — risk of additive side effects.

Other groups requiring extra caution

Melanoma.Tyrosine is a precursor of melanin, and melanoma cells actively use it. In experimental oncology, diets restricting tyrosine and phenylalanine have even been studied. There is no convincing clinical evidence of harm from supplements, but given the mechanism, people with a history of melanoma had better not take tyrosine without an oncologist's approval.

Pregnancy and breastfeeding.There are no controlled studies of the safety of high doses of tyrosine in these groups. Dietary tyrosine from a normal diet is safe, but supplements should be postponed.

Children and adolescents.Tyrosine supplements for "concentration" in studying have no evidence base in this age group; any use is possible only as prescribed by a doctor.

Hypertension, migraine, anxiety disorders.Tyrosine in studies generally did not raise blood pressure, and in the work by Deijen and colleagues (1999) a decrease in systolic pressure in cadets was even noted. However, individual reactions vary, and people in whom stimulating substances provoke migraine or anxiety attacks should start with a small amount and monitor how they feel.

Editorial conclusions

For healthy adults, L-tyrosine in the doses studied is usually well tolerated. However, hereditary disorders of tyrosine metabolism are an absolute contraindication, and phenylketonuria requires that any intake be coordinated with a metabolic center.

Taking MAO inhibitors, levodopa, thyroid and antithyroid drugs is a reason not to start the supplement on your own. The same applies to people with a history of melanoma, pregnant women and children.

If you are unsure whether you belong to a risk group, the simplest step is to show the product label to the doctor who treats you.

Related editorial materials: "L-tyrosine: available forms and which to choose," "What to combine L-tyrosine with" and "Myths about L-tyrosine."

References

1. Fernstrom JD, Fernstrom MH. Tyrosine, phenylalanine, and catecholamine synthesis and function in the brain. J Nutr. 2007;137(6 Suppl 1):1539S–1547S.
2. Chinsky JM, Singh R, Ficicioglu C, et al. Diagnosis and treatment of tyrosinemia type I: a US and Canadian consensus group review and recommendations. Genet Med. 2017;19(12).
3. van Spronsen FJ, Blau N, Harding C, et al. Phenylketonuria. Nat Rev Dis Primers. 2021;7(1):36.
4. Nutt JG, Woodward WR, Hammerstad JP, et al. The "on-off" phenomenon in Parkinson's disease. Relation to levodopa absorption and transport. N Engl J Med. 1984;310(8):483–488.
5. Jongkees BJ, Hommel B, Kühn S, Colzato LS. Effect of tyrosine supplementation on clinical and healthy populations under stress or cognitive demands — a review. J Psychiatr Res. 2015;70:50–57.
6. Deijen JB, Wientjes CJ, Vullinghs HF, et al. Tyrosine improves cognitive performance and reduces blood pressure in cadets after one week of a combat training course. Brain Res Bull. 1999;48(2):203–209.