Anybody, US citizen or not, who went to a medical school outside of the US and canada (and accredited by these countries), has to write USMLE step1, step2ck and step2cs to be eligible to start a residency. The TOEFL was abolished once they started requiring step2cs, a structured skills examination that requires a decent understanding of the english language. The exams are exactly the same as the ones US medical students have to take. The scoring system is also the same (in the old days, until the early 90s there was a separate FMGMS and FLEX exam for foreign grads. )
After finishing the three exams and after the ECFMG verifies that you actually attended a medical school accredited in the country it is located, you receive an 'ECFMG certificate' which is a pre-requistite for residency training and licensure (but not a license by itself).
www.ecfmg.org
The initial pass rate on these exams for US citizens going overseas for a medical education is indeed in the 50-65% range. After some repeat attempts, many of the people who fail initially will get certified eventually.