OK Bev,
The natural replacement is sold under a few different names depending on where in the world you live. It is a desicated animal thyroid. Usually pig now but many animals have been used over the years.
The synthetic is Synthroid or Levothroid for brand names and levothyroxin for generic. It is the same as the T4 hormone that your body produces only made in the lab. It does not have any T3 in it.
It is the same chemical make us as the natural and so technically can also be called natural.
Clear as mud??? Ask your doctor or Druggest.
Jake
On-line Facilitator