200 mcg.
That doesn't seem all that high right??? But, my "patient" was a 25 lb dog.
Since I don't know anything about how levothyroxine is handled in dogs, I called the vet to be sure we were both on the same page.
We were - the vet had been providing vet supplied levothyroxine in escalating doses. We switched the dog over to brand name Synthroid @ 200mcg/day. Over time (months) the dose reduced & stabilized at about 75 mcg - still high at first glance.
I've seen an IV dose of 350mcg, but only as a first bolus in myxedema coma. Doses above 200mcg orally generally have other issues going on - malaborption or drug interactions.