State Department spokeswoman Victoria Nuland said reports of Sotoudeh's rapidly declining health were deeply troubling, and that she had been denied medical care and kept in solitary confinement.
"We demand the Iranian government cease its intolerable mistreatment of Sotoudeh and immediately release her and the more than 30 other female political prisoners detained in Evin Prison," Nuland said in a statement.
Sotoudeh, who last month was awarded the European Union's Sakharov prize for human rights and freedom of thought, was arrested in September 2010 on suspicion of spreading propaganda and conspiring to harm state security. She is serving a six-year jail sentence in solitary confinement.
She has defended journalists and rights activists, including Iranian Nobel laureate Shirin Ebadi and Dutch national Zahra Bahrami, who was hanged in January 2011 on drug trafficking charges.
"We remain concerned for Sotoudeh's well-being given Iran's history of withholding treatment from prisoners and allowing them to die from hunger strikes," Nuland said.
Sotoudeh began a hunger strike on October 17, according to the International Campaign for Human Rights in Iran. She is protesting against a travel ban placed on her daughter and authorities' limits on visits with her family, the group said.
(Reporting by Alister Bull)