As the government bumps up against the debt ceiling, one place it cannot look for more headroom is the federal employee retirement savings program.

One of the Thrift Savings Plan’s investment options, the government securities fund, or G fund, often has been used to free up operating money for the government. However, the Treasury already has used that “disinvestment” accounting move among the other financial maneuvers it took in the spring when the government technically hit the ceiling–and whose limits are now running out.

During a disinvestment period, the Treasury stops issuing the special securities that make up the G fund, in effect taking that debt off the government’s books. The maneuver has been used many times over the years and investors are not directly affected. They continue to be credited with interest payments and the securities are reconstructed once the debt ceiling is raised.

The G fund is only one of the investment options in the TSP, however: it also has a bond fund, three stock-based funds and five “lifecycle” funds that mix investments in the five underlying funds according to the expected withdrawal date. As of the end of September, there was $375 billion in the TSP; of that $173 billion was in the G fund.

The money in the other funds, though, is not available for the government to use because of the TSP’s 401(k)-style design, in which the accounts are the personal property of the investors.

“Treasury cannot access any of the money in any of the other funds,” TSP spokeswoman Kim Weaver said in an e-mail.