The government-run mortgage financing giant, Fannie Mae lost $18.9 billion during the third quarter. The hard-to-swallow three-month deficit has led the GSE back to the Treasury Department’s door with a request in hand for another $15 billion.

Including $883 million of dividends on senior preferred stock held by the U.S. Treasury, the net loss attributable to common stockholders was $19.8 billion, or $3.47 per diluted share, Fannie Mae said. That compares to a loss of $15.2 billion, or $2.67 per diluted share, in the second quarter of 2009.

The firm explained that the third-quarter results were largely the result of $22 billion in credit-related expenses, reflecting additional dollars added to Fannie’s loan loss reserves and fair value losses associated with its purchases of loans from mortgage-backed securities trusts in order to pursue loan modifications for troubled borrowers.

Fannie Mae said in its earnings announcement that the third quarter loss resulted in a net worth deficit of $15 billion as of September 30, taking into account unrealized gains on available-for-sale securities during the three-month period.

Thus, the $15 billion price tag the GSE says it needs from the Treasury to continue its operations. The Federal Housing Finance Agency (FHFA) has requested that Treasury relinquish the funds by December 31, 2009.

Fannie Mae has been under the government’s control, with the FHFA serving as its conservator, since September of last year after its capital troubles put fear into government officials that the company’s collapse could annihilate the country’s mortgage and housing markets.

Since that time, Fannie Mae has lost a total of $111 billion. It has already received $45 billion in taxpayer dollars, and its sibling mortgage company, Freddie Mac, has taken in $51 billion in federal aid. The bailout of the GSEs has been one of the costliest government interventions in recent history.