Huseynov had been missing since January 5 when he was taken with his wife, Khanimzar, to the Internal Affairs Ministry for interrogation.
The chief doctor of the hospital told RFE/RL that Huseynov has not been diagnosed.
A source in the hospital says Huseynov is sleeping all day after being given injections. His family has not been allowed to see him.
The family's problems began when their son, Elvin, and some other teenagers were detained and charged with turning off the lights of Christmas trees in the Sadarak district of Naxcivan on December 26.
Police released the teenagers the same day after allegedly beating them and shaving their heads.