Thousands of women working for federal, state or local governments will lose all or a portion of their spousal benefit from Social Security unless they retire by the end of November, or are eligible to retire before a new pension "offset" law goes into effect Dec. 1.

Women who become eligible to retire from government service after Dec. 1 will be subject to the same government pension-Social Security offset that Congress imposed on most men five years ago.

In a move to eliminate so-called windfall Social Security benefits, Congress in 1977 enacted legislation that offset on a dollar-for-dollar basis the spouse benefit from Social Security for persons who would begin receiving a government pension after that date.

Congress added a "grandmothering" clause to the legislation, however, to give women five years before the offset applied to them. That five-year extension ends Dec. 1.

This means that women who are not eligible to retire until after that date will have their spouse benefit (a benefit earned by a husband) under Social Security subject to the same offset applying to men.

Offset is the reduction in the Social Security spousal entitlement benefit equal to the amount of the government pension the woman earned herself. The offset does not apply to any Social Security benefit an individual has earned for himself or herself, but only to the benefit earned by a spouse.

Here's an example from Social Security:

A woman working in a government job not covered by Social Security will be 65 in January 1984. She plans to retire then, and will be entitled to a government pension of, say, $150 a month. In addition, she is counting on a wife's benefit of $200 a month based on the Social Security record of her husband.

If the woman is at least eligible for her government pension before December of this year she will qualify for the full amount of both her government pension and her benefit as a spouse under Social Security, even if she does not actually retire before the cutoff date.

But if the woman first becomes eligible for her government pension in December of this year or later, then the offset will apply to her. Her Social Security spousal benefit -- $200 in this example -- will be reduced by $150, the amount of her government pension. She will get only $50 from Social Security as a spousal benefit. Her government pension benefit would not be affected.

Women who are receiving their government pension or who are eligible to retire before Dec. 1 will beat the deadline. They need only meet the age and length-of-service requirements for retirement from their government job before the cutoff; they do not have to apply for the pension by that time or actually be receiving it. They need only be eligible to receive it.

This is complicated stuff. But it is important to a large number of women in government jobs.

Social Security has a pamphlet explaining all this: SSA Publication No. 05-10007, dated November 1980. It is called "Government Pension Offset -- How It May Affect You." Check with your local Social Security office as soon as possible to see if you can meet the deadline to avoid being subject to the offset penalty. The deadline will not be extended again.