DEAR DR. DONOHUE: Can you tell me and others how you get hypoglycemia? I'm 75 and never had this before. What can be done for it other than diet, which I'm told will control it? I feel tired most of the time. I never heard of such a thing. Please advise. - Mrs. A.P.

ANSWER: Hypoglycemia is low blood sugar. With any blood sugar problem, insulin is the focus, for that's what controls blood sugar levels. Too little insulin and sugar rises. Too much and it falls.Low blood sugar occurs in two settings. If sugar is low after sleeping (fasting hypoglycemia) you might suspect a tumor of the pancreas that is causing insulin overproduction. Insulin is made in that gland. Other rare tumors, like fibrosarcoma, can also lie behind this kind of hypoglycemia. But you don't have such problems.

Your doctor told you your answer lies in diet control, implying your problem is the other kind of hypoglycemia, the reactive type. When you eat, especially sweets, your insulin release is not keyed to the sudden sugar load. Hence, the pancreas is responding with too much insulin. Sugar drops, you're shaky, out of sorts, anxious, tired.

Adhere to the diet advice you got. The multiple smaller meals low in carbohydrates and high in protein can offset the tendency toward sudden surges of insulin production I mentioned.

There is a tendency to self-diagnose this reactive hypoglycemia. Remember, anxiety can produce all the symptoms. Blood tests remove any guesswork. The low blood sugar material discusses forms and treatments. Write Dr. Donohue/No.19, Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped, self-addressed envelope and $2.

DEAR DOCTOR: I am 67. My doctor is totally unconcerned about my problems. When I lie down or get up, I get a momentary dizzy spell. I also notice that my left arm goes to sleep when I lie on it. - C.M.

ANSWER: One must always check out the possibility of nervous system disorder lying behind such dizziness, and your doctor probably has done that. Otherwise, the best explanation has to do with a quirk in blood pressure regulation.

When we change position radically, certain reflexes must occur automatically to keep blood pressure normal. If those reflexes are sluggish, as sometimes happens with age, blood pressure drops suddenly and brief dizziness ensues.

The answer sounds simple: to move slowly or by stages into the positions that cause the dizziness. For example, move from lying to sitting first rather than to an immediate standing position.

The arm problem is another common complaint of older persons. It results, as you can imagine, from pressure on arm nerves or blood vessels. Except for avoidance of the pressure situation, little can be done.

DEAR DR. DONOHUE: A friend of mine always lies down after eating. We need your opinion. Is it advisable to lie down after a meal, or should a person sit up to help food digest properly? - J.H.

ANSWER: Staying erect after a meal does help the stomach to empty faster and keeps stomach acid where it belongs, in the stomach and out of the lower esophagus (swallowing tube).

I mention this hesitantly, since most people are able to stand or lie down with impunity. If your friend has no acid problems, he can recline after eating, if that makes him feel comfortable. The Romans did it regularly, even during the actual dining. I doubt that this contributed to the decline and fall of their empire.

DEAR DR. DONOHUE: The articles I read on myasthenia gravis always mention the thymus gland. What is that and what happens to it in MG? It seems that removing it helps. Please comment. - L.I.

ANSWER: The muscle weakness of myasthenia stems from blockage of nerve signals. Imagine the brain as a radio station transmitting good signals, but with all the radio sets in its area turned off. No messages get through. That's what myasthenia is about. The fault lies with antibodies, which, in myasthenia, block nerve signals to muscles. The thymus gland is part of the immune system, programming the work of antibody-making cells.

What to do about the thymus? Well, in 10 to 15 percent of persons with myasthenia, the thymus has developed a tumor. Here, there's no alternative. The gland is removed, its duties absorbed by other parts of the immunity system.

However, about 70 percent of myasthenics have no gland tumor. Here, the choice is not as clear as with the tumor. We are not sure which patients will be helped by removal. The experts can't agree. Certainly, if the illness can't be controlled with drugs, then thymus removal may indeed aid in control.