Shaikh Saad, who suffered from colon bleeding in 1997, had been in London since August on a private visit and for medical tests. He returned to Kuwait on Wednesday.
Kuwait's amir, Shaikh Jaber al-Ahmed al-Sabah - who is also ailing - announced 10 days ago that he will take "decisive action" on matters related to his ruling family.
The 79-year-old ruler did not elaborate but spoke the same day a senior ruling family member was quoted in a newspaper as saying Kuwait needed a committee "to support the leadership".
Shaikh Salem al Ali al-Sabah, who heads the National Guards, complained to Al-Qabas daily of "chaos" and corruption in an apparent criticism of the prime minister, Shaikh Sabah al-Ahmed al-Sabah, who runs Kuwait's day-to-day affairs.
The amir said he "fully trusted" and "appreciated" Shaikh Sabah - his half brother and strong candidate to become crown prince.
The ill-health of Kuwait's two top leaders has made succession to power a public issue here. The al-Sabah's keep leadership moves in-house, but it is widely expected that Shaikh Saad, a distant cousin of the amir and prime minister, might step down.
Shaikh Jaber, who has ruled since late 1977, suffered a brain hemorrhage in 2001. He has since spent long periods in the United States and Switzerland for treatment and rest. On Monday, he was, for the first time, absent from the opening of a parliament session.