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julie- armour thyroid is a type of thyroid taken from usually animalsie pigs it containsactive thyroid t4 and t3 that are available in thenatural state and ratio. (they are standardized by iodine content). but synthroid is a t4 (levo-thyroxine) it is prepared syntheticallyin crystalline form. dose equivalents: 0.05mg l-thyroxine = about60mg thyroid (1 grain).hope this helps a little.. the armour thyroidstill contains t4 but has a little t3 in it as well. T3 does not lastas long in the body as T4. manufacturers of thyroid use different waysof standardization so do not switch from one brand to another without notifying your prescriber first. roger H