Clinical Chemistry Numericals Practice Test

The key idea is that T3 and reverse T3 are structural isomers: they have the same three iodine atoms, but their positions on the two rings are arranged differently. In T3, iodines sit at the 3 and 5 positions on the outer ring and at the 3' position on the inner ring. In reverse T3, the iodine pattern shifts so there is an iodine at the 3 position on the outer ring and at the 3' and 5' positions on the inner ring. This rearrangement changes how the molecule fits into thyroid hormone receptors and how it’s deiodinated in metabolism, which is why their biological activities differ markedly. It’s not about a different amino acid sequence (these are small molecules, not proteins), and both forms contain three iodines, so the total number isn’t the distinguishing factor. Also, reverse T3 is not completely inactive, but it has much lower activity than T3.