Abdulhadi al-Khawaja, 52, was arrested a year ago and is serving a life sentence for his role in anti-government protests that continue to roil Bahrain.
He has been on a hunger strike for about two months.
The United Nations has urged Bahrain to consider transferring the detainee, who holds Danish citizenship, to Denmark on humanitarian grounds.
"In cases where there is a hunger strike, the health and well-being of the person should be the foremost concern," United Nations spokesman Martin Nesirky said Monday.
Relatives are concerned about his health. His daughter said Al-Khawaja is having trouble breathing and is harassed by hospital staff and security guards.
"There were very long pauses," Zainab al-Khawaja said Sunday after speaking with her father. "He was trying to breathe between every word."
"His tone and the way he was speaking was like he was saying goodbye," she said. "We’re not sure if we’ll ever see him again."
Bahraini government officials said Tuesday that a medical checkup by two independent experts revealed al-Khawaja is not critically ill.
He is taking fluids orally and intravenously, and is in good condition, said Abdulaziz Al Khalifa, a spokesman for the internal affairs authority.
Rights groups such as Amnesty International have called on Bahrain to free him.
In a letter to U.S. President Barack Obama on Monday, the group and 14 others rights organizations urged Obama to demand Bahrain release him.
"The evidence is clear that al-Khawaja and others were sentenced in violation of their rights to freedom of expression, assembly and association, which are protected under international law," the groups wrote.
A judicial panel has said he does not meet the conditions to be transferred into the custody of Denmark, according to the Bahrain News Agency.
Al-Khawaja was arrested in April 2011 for his role in anti-government protests that began a month earlier with demands for political reform and greater freedoms in the Sunni-ruled, Shiite-majority nation.
In June, Bahrain found him and seven other Shiite opposition activists guilty of plotting to overthrow the country’s Sunni royal family.
He can appeal his life sentence during a hearing April 23, the government said.
