The ¥80 fall meant trading in Toshiba's shares were halted on the Japanese stock exchange.
The company said costs at Westinghouse Electric Co, which is building several nuclear reactors, had overrun.
In a statement it said it was possible goodwill could be "several billion US dollars" less than expected, resulting in a negative impact on its results.
However, it added that the figures are "still in determination".
"There is possibility of an impairment of all or part of the goodwill for both Westinghouse and Toshiba, depending on the results," it said.
Shares in the company had already begin to fall yesterday on rumours of the writedown.
"It is an unexpected development at a time when concerns had been receding," strategist Yoshinori Ogawa told the Wall Street Journal.
Moody's has downgraded Toshiba several times in the past year, citing "significantly challenged" strength in its core business.