Which thyroid hormone, despite accounting for only 5% of its total production, is more potent?

Triiodothyronine (T3) is considered more potent than thyroxine (T4) despite its lower production percentage. This potency stems from T3's stronger binding affinity to thyroid hormone receptors within target tissues, such as the brain, heart, and other organs. T3 is derived predominantly from the conversion of T4 and is responsible for regulating metabolism more effectively. Although T4 is produced in greater quantities, it is T3 that elicits a more immediate and robust physiological response in the body, influencing energy metabolism, growth, and development.

Thyroxine (T4), while the predominant form in circulation, must be converted into T3 to exert its biological effects, which highlights the crucial role of T3 in thyroid hormone action. Calcitonin, on the other hand, is not a thyroid hormone that regulates metabolism but rather plays a role in regulating calcium levels in the blood. Thyroid-stimulating hormone (TSH) is a pituitary hormone that stimulates the production of T4 and T3 but is not a thyroid hormone itself. Therefore, T3 stands out as the most potent hormone in this context.