Q. My son is 21 months old and has totally disrupted our household since he was an infant.

He is big for his age--the size his now 5-year-old sister was at 3. He pushes and shoves other kids, slaps, kicks and screams. He climbs onto kitchen counters, turns off the water heater and unscrews almost everything, even the storm door. He opens the refrigerator and climbs in.

Nothing seems to make him happy. Lately he has begun to bang his head or hit himself when he is frustrated and mad. He pulls over his highchair and anything else that isn't nailed down. There are few toys he'll play with. He's started to enjoy books, but has ripped all the pages. He loves to be outside and we try to stay out for a couple of hours every day.

We are a calm, loving family and I've tried to be more loving, talk softly and explain. I have tried every form of discipline I know, confined him to his room, made him sit in a chair, ignored his screams. I've even tried spanking him with the wooden spoon, but nothing works.

His doctor says there's nothing wrong, and that he will get better as he grows older. What am I to do in the meantime?

A. A child is born with certain tendencies -- to be outgoing or shy, anxious or easygoing, sedentary or active, and the way that child is reared defines the personality more.

A child is not, however, born to be wild, frantic, hurtful or sad and when he is, something is wrong. When parents rear one child lovingly and with obvious success but still can't find out how to make the second child happy after 21 months, there is a problem and you need a consulting opinion.

If you were to wait for your little boy to outgrow it, the stress on the family in the meantime would be terrific and many bad patterns set. Psychotherapy might help, but unless the cause is discovered, the tension would still be there.

The behavior also might not change. Hyperactivity ("attention-deficient disorder," as it's called this year) seldom disappears in the early years and in your case is made worse by two factors: Your child is so big that other children let him get away with a lot, and he's so smart he gets into even more trouble than most hyperactive children.

Hyperactivity is a symptom with many causes. Anxiety is the main one, claims Dr. Larry Silver, deputy director of NIMH. Depression is the second, generally occurring after a crisis at home or at school.

For children, however, who are hyperactive from birth -- or even before -- the causes are, says Silver, more likely to be physiological.

It's finally been documented that an exposure to lead can cause hyperactivity. One local doctor finds a severe shortage of magnesium in every hyperactive child he tests, but he doesn't know if it's the cause, the result--or if it has any connection at all. Another finds gluten a problem.

Others think it's caused by allergic reactions, with the body tossing out histamines in the central nervous system like so many sneezes.

In a related idea, some doctors such as the late Dr. Ben F. Feingold -- and many parents -- find that dyes and preservatives and salycilates (which include aspirin, many fruits and some vegetables) can make children hyperactive. Extensive studies show that perhaps only 3-5 percent of the hyperactive population reacts like this, but if your child is one, it becomes 100 percent.

The same doctors who question the Feingold Diet may acknowledge that some dyes are active compounds that can affect the nerves. And so can some dyes not permitted in foods but allowed in drugs, mouthwash and toothpaste.

The effects of our changing environment are part of the new clinical ecology field, also controversial but considered important enough for the World Health Organization to plan a conference on it next year.

This is, after all, the "chemical generation." Where babies once had cod liver oil they now get dyed vitamins. Where once their clothes were dried with sunshine, they now are dried with a strip of fabric softener -- a cause of hyperactivity in some children. Most people, but not all, can adapt to these changes, or barely respond to them.

Even the eyes might cause the problems you describe. Some optometrists say that hyperactive children may see two conflicting images at once -- which makes the children nervous -- and they claim that eye exercises may correct this (another controversial idea).

Other doctors think sugar causes hyperactivity. Dr. Keith Conners of Children's Hospital is among those investigating this area to see if the hormones released by sugar can upset a child's behavior, and if refined sugar and fructose -- such as honey -- cause different responses. Since sugar is in almost everything (including most brands of salt), this would be vital to know. Although your child is too young to qualify for his latest study, Conners is looking for hyperactive children, 6-21, whose parents would like them to participate. For more information, call 745-3247.

Most parents don't have time to wait for all the double-blind studies it will take to find the cures for hyperactivity. In the meantime, however, you should get him a multi-disciplinary checkup. The Association for Children With Learning Disabilities has a good resource list in this area and both Georgetown and Children's Hospitals have diagnostic labs.

You'll also want to do some homework to help you ask thoughtful questions and to understand the answers better.

Nutrition is probably the most important area to study, since so much is emerging here. For basic knowledge, the government has two excellent booklets, Nutritive Values of Foods ($4.50) and The Sodium Content of Your Food ($4.25), Consumer Information Center, Dept. H., Pueblo, Colo. 81009.

Some other books:

Growing Healthy by Myron Winick, M.D. (Morrow, $10.50), nutrition and nutritional problems from pregnancy to adolescence; The Sugar Trap by Beatrice Trum Hunter (Houghton Mifflin, $10.95); Sweet and Sugarfree, by Karen E. Barkie (St. Martin's Press, $5.95), a superb book of desserts without sugar, honey or artificial sweeteners, and If There's Nothing Wrong With My Child, Then What's Wrong With Him?, one of a useful series written and published by Ruth Brown and Marge Connelly, Cottage Park Publications, Box 458, Vienna, Va. 22180, $2 a copy, including postage. The Feingold Association (281-7728) also can give you information.

You also might check the Magazine Index in the Library of Congress' Periodical Reading Room for the latest information on hyperactivity.

Meanwhile, throw away the wooden spoon; spend still more hours outdoors (for your sake as much as his); give your child pure and simple foods; get some time for yourself by joining a babysitting co-op or by hiring a sitter after school, and provide as much structure for your little boy as possible. A hyperactive child needs no extra confusion, but he does need extra hugs so he won't feel like such a failure.

One thing you can be sure of: He doesn't want to be bad. He just needs you to cut those marionette strings that make him act that way.