Kuwait’s Crown Prince Sheikh Nawaf al-Ahmad al-Sabah left the United States following medical tests on Friday, state news agency KUNA reported.

The crown prince went to New York for “routine” medical check-ups 10 days ago, KUNA reported. He also went to the United States for medical check-ups last August.

Sheikh Nawaf, 80-year-old brother of the small Gulf Arab state’s ruler, underwent minor back surgery in 2013 in Germany.

A relatively low-profile figure, he became next in line to lead the US-allied oil exporter in 2006, shortly after Sheikh Sabah al-Ahmad al-Sabah became ruling emir.

Read: Kuwait’s crown prince heads to US for medical tests

The health of leading members of the ruling family is a sensitive issue in Kuwait, where less senior al-Sabah members have long been jostling for position, according to diplomats and analysts. It is not public knowledge who is third in line in Kuwait, an OPEC member state.