DEAR DR. DONOHUE: I am very concerned about my grandson. He is a healthy 18-year-old, of average weight, a non-smoker, non-drinker. What has me worried is the nosebleeds he gets at times, and rather heavy, too. Of course, he doesn't want to see a doctor about this. Should we be worried? Mrs. S.A.T.

ANSWER: Most nosebleeds in healthy young men your grandson's age are due to such things as dryness of nasal linings or irritation from chronic rubbing and itching, as from allergy, for example. A little questioning and observation relative to that might be informative here.Rarely do nosebleeds in children signify any serious condition. If they are repeated, prolonged or heavy, then you cannot ignore them, for obvious reasons. Blood clotting problems or weak nasal blood vessels are just two considerations. A somewhat unusual tumor, called angiofibroma, also might come to mind in frequent nosebleeds in a young person.

We're speaking in subjective generalities here about such things as frequency, heaviness, and so on. How often is frequent? I can't give you a definition, but any degree of frequency that gives rise to concern is "frequent" for our purposes. You are concerned. You start out your letter telling me that. That's enough to insist on some answers.

It won't take the doctor long to get a clue to any of the dire causes of nosebleeds. And the visit will put your mind at ease, if nothing else.

DEAR DR. DONOHUE: A while back, my married daughter in another city sent me an article you had written on Barrett's esophagus. I had told her I was having terrible gastric acidity and difficulty in swallowing. I never had connected the two before. My doctor is now treating my condition. P.S. - A biopsy showed there was no malignancy there, thank. Thanks for the tip. - Mrs. L.H.

ANSWER: Good news. For other readers: Barrett's esophagus, which I mentioned, is a swelling of the lower part of the esophagus from the constant irritation of stomach acid splashing upward. A symptom, of course, is Mrs. H.'s swallowing difficulty. In some, the tissue growth can be premalignant, so it has to be investigated. This is usually done via a gastroscope with biopsy tissue retrieved with the same instrument.

DEAR DR. DONOHUE: What causes water to get into the lungs? This person I know has low thyroid problems. I wonder if there is a connection between the two? What other problems can be caused by low thyroid? I will appreciate any information you can give me. - R.M.

ANSWER: If a person's thyroid production has been very low for a very long time, it is possible for fluid to accumulate in the lung covering sacs, the pleura. The name for that is pleural effusion.

Now, having said that, I must add that a person with low thyroid production would have more common and more obvious signs of the problem. I mean such symptoms as dry skin, extreme fatigue, cold intolerance and constipation. More common causes of pleural effusion are heart failure, pneumonia or other lung infections.

DEAR DR. DONOHUE: Can you answer this for me? Does the wearing of a hat cause baldness? - T.

ANSWER: Sure I can answer that. There is no reason I can think of under the sun why the wearing of a hat would cause baldness. I have had this question asked of me before. Perhaps someone can enlighten me as to where the idea originated.

DEAR DR. DONOHUE: I was wondering if you could clarify the difference between autoimmune disease and acquired immune deficiency. A co-worker mentioned that her nephew has autoimmune disease. Does this mean he got it from a parent who had acquired immune deficiency (AIDS)? Are the precautions the same for the two? - L.R.N.

ANSWER: You're confused. Please, wipe your slate clean on this matter and start over from scratch. AIDS (acquired immune deficiency syndrome) is not, repeat NOT, the same as autoimmune disease, even though both have the same initials. The confusion would be great if we started referring to autoimmune disease as "AID," for example. No one, thank heaven, ever does that. The facts are simple. Stick with them.

Both of these problems have to do with the immune system, and that is the extent of any relationship. Lots of other illnesses have to do with the immune system, but they are not AIDS either. Autoimmune diseases are, for the most part, derangements in the immune system in which the body mounts an attack on its own tissue for unknown reasons. AIDS (acquired immune deficiency syndrome) is a weakening of the immune system from infection by a virus, a known virus that's transmitted chiefly via sex or shared drug needles with a person whose blood is infected with that virus.

Most of my regular readers are quite aware of these facts, and I must ask their forbearance as I state the obvious in the above response to this question, which has come up repeatedly in my mail.

DEAR DR. DONOHUE: What does "subclinical" mean, as in subclinical hyperthyroidism? - W.A.

ANSWER: Should blood tests show an overabundance of thyroid hormones in a patient who has no outward symptoms of the problem, the hyperthyroidism is said to be subclinical. For example, blood hormone levels may be high as a result of thyroid hormone supplementation to correct an underactive gland. The blood level may not be high enough to cause symptoms (fast heartbeat, etc.), but high enough to warrant adjustment of the supplement dosage.

In short, "subclinical" means an illness is present without symptoms. Many people have had mononucleosis and never knew it.