Mohammadi was moved from her prison in Zanjan to a hospital last week after fainting twice and having previously suffered a heart attack.
On Monday, her brother, Hamidreza Mohammadi, told Norwegian NTB that the family is very worried.
"She is suffering from terrible headaches, nausea and chest pains, but it is the heart that we are most concerned about," he said.
Mohammadi, 54, was arrested in December. By then, she had already spent much of the past ten years in prison.
She was awarded the Nobel Peace Prize in 2023 for her fight for human rights.
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