Thyroid hormones regulate almost every process in the body,i ncluding body temperature,g rowth, and heart rate. They influence carbohydrate metabolism, protein synthesis and breakdown, and cardiovascular,renal, and brain function. Tw on ew polymorphs of synthetic l-thyroxine (T4) are reported and the effect of polymorphism on the solubility of this important hormone is shown. Conformational changes were also discoveredt oh ave ar emarkable effect on the strength of halogen bonding and the reactivity of the CÀI bonds,w hich could have as ignificant effect on the hormone activity.