·T3andT4 Meta bolic potency of t3 is 3-4 times that of T4. Only 20% of circulating t3 is secreted by the thyroid 3. T4 in circulation Binding form: 70% with TBG, other with alb Free form: T40.03%. T30.3%0• T3 and T4 • Metabolic potency of T3 is 3~4 times that of T4. Only 20% of circulating T3 is secreted by the thyroid • T3, T4 in circulation Binding form：70％with TBG ，other with Alb. Free form：T4 0.03%, T3 0.3%