"He is fine, but will be kept in the hospital for another two-three days," said chest physician Dr Jalil Parkar.
He refused to specify the exact ailment. "I would not discuss his medical issues," he told PTI.
Earlier in the day, BJP president Nitin Gadkari met the 86-year-old Shiv Sena patriarch to inquire about his health.
Sena sources had said that Thackeray was hospitalized only for a routine check-up.
His son and Sena executive president Uddhav Thackeray underwent angioplasty procedure at the same hospital a few day ago.
