Mechanisms of thyroid hormone metabolism in the cytosol

(A) Schematic representation of the deiodination steps involved in thyroid hormone metabolism. 5' deiodination of T4 produces the active hormone T3, while 5 deiodination produces the inactive metabolite, rT3.
(B) Simplified graphic representation of selenoprotein synthesis. The mRNA of selenoproteins contains an in-frame UGA stop codon and a stem-loop structure in the 3'UTR, Sec incorporation sequence (SECIS), which is the binding target for SBP2. SBP2 deficiency (marked with white symbol) results in impaired synthesis of all 25 selenoproteins.
Two inherited forms of thyroid hormone metabolism defects (THMDs) have been identified in humans: biallelic defects in SBP2, which cause partial deficiencies of the three deiodinases, and heterozygous defects in DIO1, which encodes D1.