Why do doctors push Synthroid?

Why do doctors push a synthetic medication over a natural dessicated thyroid medication that has been around far longer than the synthetic version?

Is it because you can't patent a natural medication...therefore pharmaceutical companies can't rake in the big bucks on them?

For more reading on this topic, read here.

I have had an endocrinologist outright refuse to prescribe a T3/T4 combination for my thyroid disease. He even went as far as to call in a prescription for Synthroid to my online prescription provider (which automatically deducts it from my bank account) after I told him I didn't want Synthroid! He's been fired.