Optional early retirement (as early as age 43 for those willing to take sharply reduced lifetime pensions) is becoming big business in government as the Carter administration reorganizes and revamps the shape of the bureaucracy.

Already thousands of government employes with 20 years of service have been given the option of leaving now on immediate pensions. More will get that choice.

Several new requests for early-retirement authority in Defense, Labor and GSA are pending before the Office of Personnel Management. OPM has the authority to allow government departments or units to offer employes the financial inducement of immediate, early retirement if they will leave and make room for others, or to minimize the impact of upcoming layoffs.

To be able to offer the unique "voluntary-involuntary retirement," U.S. agencies must be able to convince the Office of Personnel Management (OPM) that major job disruptions will be caused by reorganizations, or upcoming layoffs. The departure of long-service employes will enable agencies to make necessary job changes easier, get rid of some employes they no longer want or need, and allow them to save jobs of less senior workers who might otherwise be riffed (laid off) or bounced around because of reororganization.

In taking early retirement, employes must agree to accept substantial pension reductions, 2 percent for each year they are under age 55. For many that is too must of a financial sacrifice. For others, it is the chance to leave government with an immediate (although usually small pension) and begin a second career elsewhere.

By law, federal agencies facing reorganization or RIF may offer carly retirement on a limited basis. The Office of Personnel Management must approve the early-out. It can be limited to specific categories of workers by grade, occupation or geographical location. But OPM must authorize it; agencles cannot offer early-out on their own.

OPM was the first agency to use the new early-out authority. It granted the option to itself and the Merit Systems Protection Board because both agencies -- spinoffs of the old Civil Service Commission -- were reorganizing. That early-out authority allows workers to "volunteer" for involuntary retirement if they have 20 years of service and are at least age 50, or at any age with 25 years of service. The pension reduction of 2 percent each year applies to everybody under age 55.

In addition to approving the earlyout for its own employes, OPM has authorized it for the Department of Energy and Agriculture Stabilization and Conservation Service here, Postal Rate Commission, Panama Canal Company and some smaller units of other departments.

Agencies that have requests pending with OPM for the early-out option are the Office of Contract Compliance here of the Labor Department; headquarters, National Aeronauthics and Space Administration, and for headquarters personnel of the giant Defense Logistics Command. General Services Administration also has asked early-out retirement authority for its Federal Supply Srevice here.

Bear in mind that the requests still are pending. They have not been approved -- or disapproved -- by the OPM. They cannot take place until and unless OPM gives agencies the green light. A decision on them, one way or the other, should be coming soon.

Federal officials expect the earlyout retirement authority will be widely used this year. They anticipate requests from Defense and HEW.

Meantime, some members of Congress are becoming worried about the cost and dangers of over-use of early retirement. Even at reduced benefits, early retirement costs the civil service retirement fund extra money in longer payouts for less-than-anticipated service. And those pensions are adjusted twice yearly with the cost of living.

There are some people, on Capitol Hill and in the bureaucracy, who worry that the early-out authority may be abused by officials who want to clean house, and who use the benefits to persuade or puch employes out before they really want to go.