I would go back to the doctor and ask him - then why were thyroid symptoms less common in patients back in the 40's - 50's - 60's and so on until Synthroid was used as the drug of choice.
Armour (dessicated pig thyroid) was given to thyroid patients AND doctors then treated patients by symptoms - not on labs for many years.
I believe the medical community has made break throughs - but if a patient has no quality of life and can get some back taking a 50 year old medication - they should not be so crude in using it.
From what I know of the pig thyroid production they don't just take it from pigs willy-nilly. They do test the pigs for thyroid problems. If they didn't the FDA would surely be on their case by now. Besides, you'd have to be taking hundreds of tablets from the same pig if it had any disease, which in the processing is highly unlikely. Apparently pigs rarely get thyroid problems in the first place!
I think you mean the synthetic thyroid, not human thyroid. They definitely do not use human thyroids as a replacement for other humans. That is against a lot of moral objectors, not to mention there aren't enough dead humans to go around to provide a continual supply!
Some people believe the pig thyroid is as close to a humans hormone as can be. Many people thrive on it better than the synthetic hormone.
In the end I believe it is up to the patient to make an informed decision as to what they want to do.
Hope that helps a bit!