Shares of struggling Thornburg Mortgage dropped 13 cents Tuesday morning after the firm said it had received margin calls that threaten a planned restructuring.

Margin calls require companies that have borrowed money to provide more collateral for financing those loans.

In the statement released late Monday, Santa Fe-based Thornburg Mortgage said its lenders "have withheld funds payable to the company," an unexpected move that was in conflict with a lending agreement.

The statement also says that lenders "continue to withhold virtually all cash owed to the company."

If Thornburg Mortgage cannot reach a satisfactory agreement with its lenders, "the company would not be able to pay its debts as they become due in the usual course of business, or the company's total assets would be less than the sum of its total liabilities," the statement said.

In the statement, Thornburg Mortgage also said it is extending the expiration date of its ongoing tender offer for its preferred shares until Sept. 23, 2008.

Thornburg Mortgage also is asking bondholders to accept new debt in lieu of a Sept. 30 interest payment.

Thornburg Mortgage's shares were at 22 cents per share in noon-time trading on Wall Street.

The company originates jumbo mortgages, those worth more than $417,000, but Thornburg Mortgage has temporarily stopped issuing those mortgages.