<prompt> Thyroxine is synthesized from:PhenylalanineTryptophanTyrosineNone of the above  <response>: In dopaminergic cells in the brain, tyrosine is conveed to levodopa by the enzyme tyrosine hydroxylase TH.TH is the rate-limiting enzyme involved in the synthesis of the neurotransmitter dopamine.In the adrenal medulla, tyrosine is conveed into the catecholamine hormones norepinephrine noradrenaline, and epinephrine.The thyroid hormones triiodothyronine T3 and thyroxine T4 in the colloid of the thyroid also are derived from tyrosine. 