UPDATE 2-Sallie Mae has 2nd-qtr loss, hurt by writedowns

* Quarterly loss tied to derivatives, hedging

* "Core" results top forecasts

* Shares fall 6.5 percent after-hours (Adds "core" earnings topping forecasts, after-hours share movement)

NEW YORK, July 21 (Reuters) - Sallie Mae SLM.N, the largest U.S. student loan provider, on Tuesday said securities and loan write-downs and disruptions to credit markets led to a second-quarter loss, and its shares fell after-hours.

The loss for the Reston, Virginia-based company was $123 million, or 32 cents per share, and compared with a profit of $266 million, or 50 cents, a year earlier. The most recent period included a $484 million loss tied to derivatives and hedging.

Sallie Mae, whose formal name is SLM Corp, said "core" earnings rose 9 percent to $170 million, or 31 cents per share. On that basis, analysts on average had expected profit of 6 cents per share, according to Reuters Estimates.

Sallie Mae shares nevertheless fell 6.5 percent to $8.89 in after-hours trading. They had fallen 28 cents to $9.51 in regular trading.

Results included a gain of 44 cents per share from debt repurchases, and a loss of 13 cents per share related to disruptions in the commercial paper market. Student loan volume increased 53 percent to $3.7 billion.

Sallie Mae said it set aside $362 million for loan losses, up from the first quarter's $297 million, while charge-offs on managed private education loans more than doubled from the first quarter to $355 million.

Results were announced hours after the House Education Committee endorsed an Obama administration proposal to reshape the $92 billion U.S. college student loan market.

A proposed law would shift much of the lending into a direct loan program and by July 2010 end the Federal Family Education Loan Program, once a big source of profits for Sallie Mae, Student Loan Corp STU.N and other rivals.

Citigroup Inc (C.N) owns an 80 percent stake in Student Loan Corp, regulatory filings show.

Analysts expect the full House of Representatives to pass the law before the chamber's August recess.

Sallie Mae and three rivals did win a key U.S. Department of Education mandate in June, capturing a five-year contract to service $550 billion of federal student loans.

Shares of Sallie Mae began the year at $8.90. The company announced results after U.S. markets closed. (Reporting by Jonathan Stempel; editing by Carol Bishopric)