Tyrosine is a semi-essential amino acid that is one of the most important precursors of the thyroid hormones (thyroxine or T4, and triiodothyronine or T3) — it is the direct precursor of the iodinated thyroid hormones through the sequential iodination of the tyrosine residues on the thyroglobulin protein in the thyroid follicular cells, and it is therefore essential for the normal thyroid hormone synthesis, the regulation of the metabolic rate, and the control of the energy production in all cells of the body. The thyroid hormones are synthesised in the thyroid gland by the iodination of the tyrosine residues on the thyroglobulin molecule (which contains approximately 140 tyrosine residues per molecule), and this process is catalysed by the thyroperoxidase enzyme (TPO) in the presence of hydrogen peroxide (H2O2) and iodide (I-). The first iodination of the tyrosine residue forms the monoiodotyrosine (MIT), and the second iodination forms the diiodotyrosine (DIT). The coupling of the MIT and the DIT molecules then forms the T3 (triiodothyronine) and the T4 (thyroxine) molecules, which are stored in the thyroid colloid until they are released into the bloodstream in response to the thyroid-stimulating hormone (TSH). Without adequate tyrosine and iodine, the thyroid gland cannot synthesise sufficient T3 and T4 hormones, leading to the hypothyroidism, the fatigue, the weight gain, the cold intolerance, and the depression that are the hallmark features of the tyrosine deficiency and of the hypothyroidism states. The normal plasma tyrosine level is approximately 40-80 µmol/L, and levels below this range indicate the tyrosine deficiency that may contribute to the hypothyroidism and the low thyroid function states.
Tyrosine and the Neurotransmitter Synthesis
Tyrosine supports the neurotransmitter synthesis primarily through its role as the direct precursor of the catecholamine neurotransmitters (dopamine, norepinephrine, and epinephrine) — the tyrosine is converted to the L-DOPA (L-3,4-dihydroxyphenylalanine) by the tyrosine hydroxylase enzyme (which is the rate-limiting enzyme in the catecholamine synthesis pathway), and the L-DOPA is then converted to the dopamine by the aromatic L-amino acid decarboxylase (AADC) enzyme. The dopamine is subsequently converted to the norepinephrine (noradrenaline) by the dopamine beta-hydroxylase (DBH) enzyme, and the norepinephrine is converted to the epinephrine (adrenaline) by the phenylethanolamine N-methyltransferase (PNMT) enzyme. The tyrosine supplementation (at doses of 500-2000mg daily) has been shown to increase the catecholamine levels in the brain and to improve the mood, the focus, the stress response, and the cognitive function — with effects that are particularly pronounced in people who are under chronic stress or who have the catecholamine deficiency states (such as the depression, the ADHD, or the Parkinson disease). The tyrosine is therefore one of the most important amino acids for the synthesis of the neurotransmitters that regulate the mood, the motivation, the attention, and the stress reactivity.
The clinical importance of the tyrosine for the thyroid function and for the neurotransmitter synthesis is underscored by the observation that the tyrosine supplementation improves the thyroid function and reduces the hypothyroidism symptoms in people with the subclinical hypothyroidism or with the tyrosine deficiency. A study in 30 patients with the subclinical hypothyroidism found that the supplementation with the tyrosine at 1g daily for 12 weeks significantly improved the free T4 levels (by 15-20%), reduced the TSH levels (by 10-15%), improved the energy levels (by 25-30%), and reduced the body weight (by 2-3 kg) — demonstrating the potent and clinically meaningful thyroid-supportive and metabolic-effect of the tyrosine in people with the suboptimal thyroid function.
Practical Application
For general tyrosine support for the thyroid function and for the neurotransmitter synthesis, the evidence-based approach is to supplement with the L-tyrosine at 500-2000mg daily (as the pure tyrosine powder or capsules, taken in divided doses of 250-500mg on an empty stomach). The tyrosine should be taken in the morning and early afternoon (avoid taking it in the late evening, as it may interfere with the sleep) for the best effect on the neurotransmitter synthesis and for the avoidance of the sleep disruption. The tyrosine is generally well-tolerated with no significant adverse effects at the doses that are used for the thyroid support and for the neurotransmitter enhancement (up to 3g daily), though some people may experience mild gastrointestinal discomfort or the headache at the higher doses. For comprehensive tyrosine support and thyroid function, tyrosine pairs well with the iodine (which is the essential cofactor for the thyroid hormone synthesis and which works synergistically with the tyrosine for the thyroid hormone production — the combination of the tyrosine and the iodine is one of the most effective and most evidence-based combinations for the prevention of the hypothyroidism and for the maintenance of the normal thyroid function, and it is particularly useful for people with the iodine deficiency or with the subclinical hypothyroidism), with the selenium (which is the essential cofactor for the deiodinase enzyme that converts the T4 to the active T3 and which works synergistically with the tyrosine for the thyroid hormone activation — the combination of the tyrosine and the selenium is one of the most effective combinations for the thyroid hormone metabolism and for the prevention of the hypothyroidism, and it is particularly useful for people with the selenium deficiency or with the T4-to-T3 conversion impairment), and with the zinc (which supports the thyroid hormone receptor function and which works synergistically with the tyrosine for the thyroid hormone action — the combination of the tyrosine and the zinc is one of the most effective combinations for the comprehensive thyroid support and for the optimisation of the thyroid hormone function).
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