Jared Followill tweeted: "Dallas, I cannot begin to tell you how sorry I am. There are internal sicknesses & problems that have needed to be addressed. No words."
He added: "I love our fans so much. I know you guys aren't stupid. There are problems in our band bigger than not drinking enough Gatorade."
However, the statement released by the band did not address other troubles, and it said Caleb "suffered from heat exhaustion and dehydration ... causing his vocal chords to seize."
The Nashville-based band also canceled Saturday's show in Houston. That show was rescheduled for Sept. 22.
Nekesa Mumbi Moody is the AP's music writer. Follow her at http://www.twitter.com/nekesamumbi