Before the FDA did approval on these drugs, synthroid had the corner. With the approval process, the drugs had to show their stability across the different dosages. Unithroid was the first and synthroid sued the FDA to not have to go thru the process, they wanted to be Granfathered in. Their product was shown to not be stable ie. each tablet could have a different amt of T4 in it. After 1-2 yrs, synthroid gave in and applied and did all the testing and improved their product. I took Synthroid first also and wondered why I felt good sometimes and then did poorly. After this all started I then knew it was the Synthroid. So I switched to the Unithroid and felt so much better!
Very interesting, thank you for the explanation.