Jeffs was hospitalized Sunday after lapsing into a coma while fasting, officials said.
Jason Clark, a spokesman for the Texas Department of Criminal Justice, said Jeffs was sent to a hospital in Tyler on Sunday night and was in critical but stable condition Monday.
"He was not eating or drinking enough fluids and also has some other medical conditions," Clark said without elaborating, citing inmate privacy rules.
Jeffs told prison officials he was not on a hunger strike but that he had been "fasting," Clark said.
Jeffs won't be eligible for parole for 45 years.
He must serve at least 35 years of a life sentence on one of the child sex charges, and at least 10 years on the other.
Jeffs, who represented himself, was charged with two counts of sexual assault on a child stemming from a 2008 raid on a ranch his church operates near Eldorado, Texas.
Jeffs was on the FBI's 10 Most Wanted list when he was arrested five years ago during a routine 2006 traffic stop in Las Vegas.
He was convicted in Utah on two counts of being an accomplice to rape for using his religious influence over his followers to coerce a 14-year-old girl into marrying her 19-year-old cousin. Afterward, he was sentenced to two consecutive prison terms of five years to life.
But in July 2010, the Utah Supreme Court overturned his convictions, ruling that the jury instructions were erroneous.
Utah Attorney General Mark Shurtleff said last week Utah is prepared to retry Jeffs, depending on the outcome of the Texas case.
The Texas legal proceedings were set off after about 400 children were taken from the sect's Yearning for Zion ranch in 2008.
Jeffs was also charged with bigamy after the raid and is expected to be tried on that charge later.
Child protection officials said they found a "pervasive pattern" of sexual abuse on the ranch through forced marriages between underage girls and older men.
But the Texas Supreme Court ruled the state had no right to remove the children.
The court also said the state lacked evidence to show that the children faced imminent danger of abuse.
Most of the children were returned to their families, although some men at the ranch were charged with sexual abuse.