Ten of 20 percent of the elderly senile do not really suffer from dementia - progressive loss of memory and brain function - but from other conditions and diseases that could be treated if doctors only recognized them, a group of experts on aging agreed last week.

This means, it was agreed, that 300,000 to 600,000 of the estimated 3 million Americans who show symptoms of brain failure are mistakenly labeled as "senile," and, as a result, often go untreated in nursing homes and mental hospitals.

"The prospect of 300,000 doomed people in the United States today who could have been restored to useful life by appropriate evaluation and treatments is staggering and demands action," said Dr. Richard Besdine of Harvard Medical School and the Hebrew Rehabilitation Center for the Aged in Roslindale, Mass.

Besdine was author of a draft report made by a six-member task force for discussion by the 30 specialists who attended a two-day Conference on Treatable Diseases in the Elderly at the National Institute of Health in Bethesda.

One of the largest single causes of false senility, it was agreed, is drug intoxication: that is, reactions to medications. The most commonly guilty medications are tranquilizers, and among these the most common is Valium, the nation's most often used prescription drug.

Among treatable and often unspotted physical diseases that can cause brain dysfunction and mimic truly irreversible senility are heart disease strokes, infections (including pneumonia), anemia, nerve diseases, brain tumors and blood clots, kidney or liver failure, metabolic disease, strokes infections (including pneumo-nutritional deficiencies (sometimes caused by faulty diet), reactions to chemicals and alcoholism.

The experts' consensus on all these points was reached at the conference, which ended last Wednesday.

The gathering was one of a new series of NIH "consensus meetings" that assemble specialists to make recommendations to the medical world in newly emerging, often puzzling areas.

The idea for the meetings - the creation of Dr. Donald Fredrickson, NIH director - came from a 1977 conference on mammography, or breast X-raying. Because such X-rays may sometimes trigger as well as find cancers, the experts recommended limiting mammography to women with a far greater chance of benefiting than suffering any harm, meaning women over 50, women 35 to 49 who have had one breast cancer already and women 40 to 49 who have had mothers or sisters with breast cancer.

The breast conference was one NIH was virtually forced to hold to reassess the controversial X-ray method that NIH itself was widely funding.

But NIH has also been under the fire of critics, including members of Congress, for not doing enough to translate research findings into everyday care. The consensus conferences are one reply.

The specialists on aging and related fields gathered by NIH's National Institute on Aging included Dr. Ernest Gruenberg, chairman of mental hygiene in the Johns Hopkins School of Hygiene and Public Health; Dr. Robert Katzman, neurology head at Albert Einstein Medical College, New York, and Dr. James Baker, the Veterans' Administrations' associate chief for mental health treatment.

The discussions will be translated into recommendations in coming months. But there was general agreement to tell practicing doctors that:

There are indeed irreversible causes of "chronic brain syndrome," or dementia, such as repeated blood vessel blockages and various brain and nerve diseases.

They would be well advised, however, to spend more time thoroughly and aggressively examining and testing their confused or disoriented patients to exclude reversible causes.

Deep psychological depression, too, can be classed as a common and often treatable, reversible cause of apparent senility.

It is "amazing," said one doctor attending the conference, how often even a heart attack can occur in silence with no outward physical sign other than mental confusion.

"We're talking about a large group of underlying, unrecognized diseases that are usually improvable, often reversible and sometimes completely cureable," said Besdine.

To say that 10 to 20 percent of the supposed senility cases have potentially reversible causes does not mean other forms of senility shouldn't be treated too, said Dr. Robert Butler, director of the National Institute on Aging.

"In all cases," he said, "the earlier the treatment, the better. The brain doesn't do very well when it is ignored over a long period of time."

Dementia is not a reason "for locking up the patient and throwing away the key," said Dr. Carl Eisdorfer, University of Washington psychiatry head.

The treatment may sometimes be as simple as withdrawing a drug or giving a drug. But often, said conferees, it must mean finding family or other "societal support" for the patient. Doctors and other health workers must themselves get out of their offices into patients' homes before they can understand their problems, the conferees agreed.

"People fear few things more than losing their minds and being 'put away' in a nursing home", said another National Institution on Aging report. Yet too many conditions are "mislabeled as 'senility'" simply because the patient is old, it said.