Flexi Says: There are two main thyroid hormones produced by the follicles: thyroxine (T4) — which contains four iodide ions and is represented by the structural diagram below — and triiodothyronine (T3), which contains three iodide ions. T3 is much more powerful than T4, but T4 makes up about 90 percent of circulating thyroid hormone, and T3 makes up only about ten percent. However, most of the T4 is converted to T3 by target tissues.
