Two thyroid hormone molecules are in the image below from the back cover of The Metabolic Treatment of Fibromyalgia. In the upper left-hand corner is a T4 (thyroxine) molecule. The four red spheres represent the four iodine atoms of the T4 molecule. The iodine atoms are attached to the "core" of the molecule. The core consists of the amino acid tyrosine with three oxygen and two nitrogen atoms attached to it. Many researchers believe that T4 is a storage form of thyroid hormone that has no biologic functions. Other researchers disagree. Among those who believe that T4 has some biologic functions are the two leading researchers studying the relation of metabolism to fibromyalgia: Dr. John C. Lowe in the USA and Professor J. Eisinger in France.

The larger molecule, at the from of the image, is a T3 (tri-iodothyronine) molecule. T3 is the most metabolically-active thyroid hormone. The three red spheres attached to the molecule represent the three iodine atoms of the T3 molecule. T3 is produced when an enzyme separates one of the iodine atoms from the T4 molecule.  The enzyme, called 5'-deiodinase, specifically separates from the molecule the 5' (pronounced "5-prime") iodine atom. The red sphere in the foreground at the bottom of the drawing represents the 5' iodine atom.