The Kadapa Constituency MP was shifted to the Nizam's Institute of Medical Sciences (NIMS) late on Friday night for better treatment.
Reddy is fasting in protest against the bifurcation of Andhra Pradesh without giving justice to all regions.
He is currently under judicial custody in connection with a disproportionate assets case.
He was earlier being treated at the Osmania General Hospital.
According to media reports, eight doctors are monitoring Reddy in the emergency block of NIMS.
The reports quoted NIMS doctors, as saying that his sugar and blood pressure levels have reduced considerably, while the ketone bodies in his urine are high.
Doctors have warned that his condition may further deteriorate if he persists with his fast.
Members of his family, including mother Y S Vijayamma, wife Bharati and brother-in-law Anil Kumar have not been allowed to meet him up close due to the possibility of infection.
Jagan Reddy is only drinking water since he began his fast in the Chanchalguda Central Jail last Sunday.
His family and other party leaders have appealed to him to call off his hunger strike, but to no avail.
His mother and wife have filed a memo before a CBI court, seeking permission for one of them to stay with Jagan in the hospital.
The plea will be heard today.
Reddy has been in jail since May 2012.
--ANI (Posted on 31-08-2013)