The stock fell as much as 26 percent earlier in the day, its biggest intraday decline in more than four decades.
The Tokyo-listed shares dived 16 percent to end the day at 242.3 yen after Japanese media said the vast conglomerate could book losses of as much as 700 billion yen ($6.1 billion) in the US unit.
The stock fell as much as 26 percent earlier in the day, its biggest intraday decline in more than four decades, according to Bloomberg.
The eye-popping loss would force the company, which has been struggling to move past an embarrassing accounting scandal, to seek support from a government-backed lender, the Nikkei newspaper said.
Toshiba, one of Japan's best-known firms, said exact figures were not finalised.
"We are still discussing how to deal with this issue, and no concrete decisions have been made," it said in a statement Thursday.
Toshiba has fallen more than 45 percent since the company, which makes everything from nuclear reactors to laptops, warned in late December of a possible one-time loss on its US nuclear business.
It said costs linked to the acquisition last year by its US subsidiary Westinghouse of a nuclear services company could come to "several billion US dollars" and dent its earnings.
Japan's public broadcaster NHK said Thursday Toshiba had drafted a plan to sell more assets and delay expensive plans such as layoffs to boost its coffers by about 300 billion yen.
Toshiba might spin off its semiconductor operations into a separate firm, among other efforts, in a bid to shore up its finances, NHK and the Nikkei said.
Toshiba said Wednesday that splitting its memory chip business was an option, but that no conclusion had been reached.
"There is a bigger sense of crisis than you would have imagined even a week ago," said Damian Thong, an analyst at Macquarie Group.
"It seems like they are scrambling for a solution, something to announce to shareholders on the day of their next financial update," Thong told Bloomberg News.
Japan's earning seasons gets into full swing later this month but Toshiba has yet to announce when it will release its latest results.
Toshiba's nuclear woes are the latest blow to the once-proud pillar of corporate Japan, most notably a profit-padding scandal in which bosses for years systematically pushed subordinates to cover up weak financial results.
In the aftermath, the company has been shedding businesses and announced the sale of its medical devices unit to camera and office equipment maker Canon.
Last month, Standard & Poor's cut two of Toshiba's credit and debt ratings by one notch each in response to the potential nuclear losses.