Next month, all investment restrictions will end in the $7 billion tax- deferred thrift savings plan, the government version of a 401(k) plan. The change means federal workers can invest in any or all of the plan's options: the G-fund (treasury securities), C-fund (stock market) or the F-fund (bond market).

Since the program started, employees in the Civil Service Retirement System had to put their payroll deduction contributions in the G-fund. Workers in the Federal Employees Retirement System could put part of their contributions in the stock or bond options; but the rest, plus matching government contributions, had to go in the treasury option.

During the open season that ends Jan. 15, employees can reallocate their investments. They also can take money from their treasury accounts and put some or all of it in the two higher-risk options.

People under the FERS pension plan who already are contributing to the stock or bond market funds must submit a new election form (called the TSP-1) to their agency before the first January pay period starts. If they don't do that, all future contributions to their account will go to the treasury fund option.

The stock fund is worth about $156 million. The bond fund is worth about $48 million. The treasury fund is worth more than $7 billion.

Obviously, the stock and bond market funds go up and down with the market. Money in the treasury option is invested at a guaranteed rate that can change monthly. In 1987 the treasury fund had an average annual rate of 9.08 percent. In 1988 that rate was 9.19 percent. Last year it was 9.01 percent. This year the rate is 8.97 percent, and it will be 8.375 percent in January. Moonlighting Ban

The Senate Governmental Affairs Committee will hold hearings early next year on a portion of the Ethics Reform Act that, as of Jan. 1, makes it illegal for federal workers to be paid for occasionally writing articles, speaking or teaching off-duty. The ban does not affect employees who have contracts or who are paid salaries for writing, speaking or teaching on a regular basis. It covers only activities that employees engage in irregularly, payment for which is considered honoraria, rather than salary.

Committee Chairman John Glenn (D-Ohio) has said he will review the ban as soon as possible. Many workers think it violates their rights. Many like moonlighting and need the money. But until Congress changes the law -- if it does -- everyone should be careful about paid off-duty activities. The maximum penalty for breaking the ban is a $10,000 fine. Retiring Types

Lois McCabe, a personnel security specialist, is retiring next month from the Naval Security Group. She has 33 years of federal service, including two years as a WAVE during World War II.

Goddard Space Flight Center in Greenbelt is losing its favorite accounting technician, Goldie Lusby, after 28 years with the federal government.

Frances S. Smith will retire this month from the Securities and Exchange Commission's division of interest management. She has been with the government for 28 years.