Thyroxine biosynthesis iodine incorporation

A noteworthy example of electrophilic aromatic substitution in nature, as mentioned in the introduction, is biosynthesis of the thyroid hormone thyroxine, where iodine is incorporated into benzene rings that are derived from tyrosine. 

Thyroxine (see the model above ) is an aromatic compound and a key hormone that raises metabolic rate. Low levels of thyroxine (hypothyroidism) can lead to obesity, lethargy, and an enlarged thyroid gland (goiter). The thyroid gland makes thyroxine from iodine and tyrosine, which are two essential components of our diet. Most of us obtain iodine from iodized salt, but iodine is also found in products derived from seaweed, like the kelp shown above. An abnormal level of thyroxine is a relatively common malady, however. Fortunately, low levels of thyroxine are easily corrected by hormone supplements. After we study a new class of reaction in this chapter called electrophilic aromatic substitution, we shall return to see how that reaction is related to thyroxine in "The Chemistry of... Iodine Incorporation in Thyroxine Biosynthesis."... 

Biochemical iodination, as in the biosynthesis of thyroxine, occurs with enzymatic catalysis. Thyroxine biosynthesis is discussed further in The Chemistry of. .. Iodine Incorporation in Thyroxine Biosynthesis box in Section 15.1 IE. 

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