Thyroid biological activity, triiodothyronine

In thyroxine (//.//) and the more biologically active triiodothyronine, the iodine atoms in the 3- and 4- positions force the two rings into a conformation in which they are perpendicular to one another. Further work, with analogues, indicates that this arrangement is essential for thyroid function (Dietrich et al. 1977). 

Triiodothyronine (3, 5,3-L-triiodothyronine, T3) is a thyroid hormone. It is producedby outer ring deiodination of thyroxine (T4) in peripheral tissues. The biologic activity of T3 is 3-8 times higher than that of T4. T3 is 99.7% protein-bound and is effective in its free non-protein-bound form. The half-life of triiodothyronine is about 19 h. The daily tur nover of T3 is 75%. Triiodothyronine acts via nuclear receptor binding with subsequent induction of protein synthesis. Effects of thyroid hormones are apparent in almost all organ systems. They include effects on the basal metabolic rate and the metabolisms of proteins, lipids and carbohydrates. 

Type-1 iodothyronine deiodinase (ID-1) is an enzyme containing a selenocysteine residue in its active site that catalyzes the conversion of a human thyroid prohormone (thyroxine T4) to a biologically active hormone (3,5,3 -triiodothyronine T3) through 5 -deiodination (Schone 11.29). ... 

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