Turning the thermostat down to 65 degrees or less may save energy, but it can also be fatal for persons 65 years or older, according to the National Institute on Aging.

A condition known as hypothermia, which normally affects younger, healthy persons exposed to cold weather for prolonged periods at relatively high temperatures.

Little research has been done here on hypothermia or subnormal body temperature. Estimates in Great Britain, where more study has been given to the problem, are that anywhere from 500 to 10,000 older persons die because of hypothermia every winter.

The most sinister aspect of hypothermia is that persons most susceptible to it are also least aware that their bodies are losing heat, according to Dr. Richard Besdine, specialist in geriatrics who serves as a consultant to the National Institute on Aging.

"Unfortunately," Besdine said, "people who are more susceptible perceive cold less well." One of the causes of hypothermia is a failure of the blood vessels to constrict, a normal reaction of the body heat. When blood vessels do not constrict, the result may be rapid loss of heat.

As the body's temperature drops, Besdine said, there is a "clouding of perceptions and decreased alertness, sleepiness and lethargy. The problem gets more severe as the body temperature declines."

According to Besdine, older persons "have less good temperature regulation," making them more susceptible to both heat and cold.

Among the symptoms are bloated face, lack of shivering - a normal response to cold - low blood pressure, drowsiness which could lead to a coma, slowed heartbeat, slurred speech and low body heat, each of the other symptoms might be confused with other ailments such as stroke, diabetic coma or a heart condition.

Since standard medical thermometers in this country usually do not measure temperatures below 94 degrees Fahrenheit, detecting hypothermia by measuring body heat may be difficult. According to a spokeswoman for the Institute, hypothermia is serious "but treatable" at body temperatures above 90 degrees. Below 90 she said, "it's a medical emergency."

Once hypothermia sets in - at a point of 95 degrees or less - treatment is delicate. A too-rapid increase in body temperature may result in the person going into profound shock, according to Besdine. Although the reasons are not well understood, Besdine said the theoretical explanation for shock is cold surface blood being dumped into the central circulatory system. Besdine said body temperatures should be raised no more than one degree an hour.

Among persons 65 or older, those most susceptible to hypothermia are persons with diabetes that is out of control, kidney failure, congestive heart failure, Addison's disease, low blood sugar or arthritis, or people who have had a stroke, according to Besdine. Stroke poses a "double jeopardy," he said, because it not only immobilizes a person but it "can affect the body heat regulating mechanism."

Any disability that keeps a person immobile increases the chances of hypothermia. "If you can't get around, you don't make as much body heat so that even small losses of body heat can be serious," Besdine said.

Additionally, older persons taking certain tranquilizers, such as phenothiazines - or other medications are more susceptible to hypothermia.

"Undiagnosed hypothermia is an unrecognized contributor to hospitalizations and death in older people," Besdine said. "There are often multiple problems in the elderly and, when there are, each affects the other adversely."

The cause of death from hypothermia is usually a heart attack triggered by low body temperatures.

Prevention of hypothermia is simple enough, according to Besdine. Room temperatures should be maintained "certainly above 65 degrees and for the at-risk [susceptible] elderly 68 or 70 degrees." In addition, Besdine said, older persons should wear warm clothing inside and "even things foolish-sounding like a muffler, mittens, a hat and warm socks."