Scientists have discovered a genetic defect that appears to cause deficit-hyperactivity disorder, more commonly known as hyperactivity. People afflicted with this disorder have trouble focusing their attention and controlling impulses, and tend to be rest-less and aggressive. The defect occurs in a gene that regulates the body's use of the thyroid hormone. This finding was made in continuing study organized by the National Institute of Diabetes and Digestive and Kidney Diseases in which 70 percent of children inheriting the gene were hyperactive while only 20 percent of the children not inheriting the gene were hyperactive. And although the researchers admit that this defective gene probably accounts for only a fraction of cases of hyperactivity, this discovery will allow doctors to spot newborn babies likely to have the disorder, and also to provide clues for its cause and treatment.