Answer to Question #61903 in Human Anatomy and Physiology for Sanjukta Ghosh
Is Thyroxine a hormone?
I have read the phrase 'T3 is three to four times more potent than T4' several times both in books and websites. But isn't T4 a prohormone of T3? Why do then these sources talk about its potency as a hormone(I suppose)?
Indeed, T4 is a prohormone to T3. But there is an equilibrium between T3 and T4 content in a thyroid gland – that means that not every molecule of T4 is converted to T3. Both T3 and T4 are secreted into the blood and both have some biological activity (it is natural that T3, as a finished form, have greater effects on the body). That means that both T3 and T4 can be considered as hormones and thus we can compare their activity.