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First, the thyroid gland absorbs iodine, a trace mineral, from food. Then, the iodide is oxidized by thyroid peroxidase (TPO) into active iodine. Once oxidized, these iodine molecules are ready to attach to the tyrosine found in thyroglobulin (a protein produced by the thyroid gland). When iodine and tyrosine join together, they create the thyroid hormone precursors, monoiodotyrosine (T1), and diiodotyrosine (T2). T1 and T2 then combine to form thyroid hormones triiodothyronine (T3; thyroglobulin with three iodine molecules) and thyroxine (T4, thyroglobulin with four iodine molecules). T4 and T3 are the primary thyroid hormones, essential for regulating metabolic processes throughout the body.
Supplementing with l-tyrosine can help to increase the production of the adrenal hormones. Your adrenal glands send dopamine, norepinephrine, epinephrine into your blood when you're physically or emotionally stressed. This response can cause a depletion of their levels. Elevating brain tyrosine concentrations may stimulate the production of these "feel-good" neurotransmitters, which helps to boost mood, lower stress, and correct adrenal depletion.
A low level of thyroxine (T4) and high thyroid-stimulating hormone (TSH) levels indicate an underactive thyroid. Supplementing with l-tyrosine may lead to a reduction in TSH. This reduction is because tyrosine is a building block for the thyroid hormones, so supplementing it can produce more thyroid hormones. People with an overactive thyroid, like hyperthyroidism or Graves' disease, should avoid tyrosine because it might affect already high thyroid hormone levels.
L-tyrosine is a precursor of several of the neurotransmitters affected by depression—including adrenaline, dopamine, and noradrenaline (NA). Two clinical studies on patients with depression and healthy volunteers show that treatment with l-tyrosine positively supports depression management.
Tyrosine increases dopamine levels in the brain and can improve mental performance in stressful situations. One clinical study asked 22 healthy adults to switch between two different tasks rapidly. Compared to the control group, tyrosine positively promoted cognitive flexibility—a brain function assumed to be regulated by dopamine.