Synthetic T4 contains the active ingredient levothyroxine sodium. Brands include Levothyroxine, Synthroid, Levoxyl...
Natural desiccated thyroid extract is made from dried animal glands (typically from a pig - porcine). Brand names include Armour, Naturethroid, Thyroid WP...
I take Eutroxsig (synthetic T4) for Hashimoto's thyroiditis but I had read good things about Armour so I tried it. I wasn't absorbing this medication so I had to switch back to synthetic. I do fine on synthetic though.