Caterpillar Tractor Co. said yesterday it will lay off 2,450 workers in Illinois and Iowa starting in December, and will temporarily shut down several plants beginning next month.

The heavy construction-equipment manufacturer had a U.S. work force of 47,564 on Sept. 30, the company said. That was up from 43,666 domestic employes at the end of 1983 but down from more than 60,000 during the peak-employment period of mid-1979.

In a one-page statement announcing the actions, Caterpillar said the layoffs and temporary plant shutdowns were needed "to bring production into line with current and projected sales."

Caterpillar's directors last week, in slashing dividends on common stock by two-thirds, hinted that layoffs and plant consolidations or closings would be necessary because earlier expectations of a return to profits in 1984 had fallen through. The company reported yesterday that its third-quarter loss widened to $92 million on sales of $1.71 billion, up from $1.63 billion a year earlier.

Caterpillar had posted a $24 million profit in the second quarter, its first quarterly profit since mid-1982.

The third-quarter loss lifted the company's year-to-date deficit to $177 million on sales of $4.92 billion, compared with a nine-month loss of $334 million on sales of $3.7 billion a year ago.