Will a US passport-holding physician with postgraduate specialty?
You would need to check with US authorities. All new certifications and licenses are subject to current board certification and licensing requirements. The likely outcome of a non-US trained physician with a US passport is that that physician would be treated just like a citizen from another country with that same training and education. This likely means you would not be eligible even for a medical license. Standards change over time. The current standards likely apply. So although years ago you may have qualified, now you probably do not.