DEAR DR. DONOHUE: I have just read three articles on hypertension (high blood pressure). I have yet to see anything on hypotension (low blood pressure). I have it. I think it might be due to the stomach surgery I had to lose weight. I have lost 210 pounds in 18 months. When my pressure drops, I feel I am about to pass out, and I have to sit down. I have no trouble when I sit. What about those of us with low blood pressure? – S.S.

ANSWER: People whose blood pressure is lower than the accepted normal pressure often lead longer lives than the rest of the population. However, when low blood pressure causes symptoms, that’s another matter. If the systolic pressure (the first number of a blood-pressure reading) drops much below 90, the heart has trouble getting enough blood to the brain. People feel lightheaded, dizzy and on the verge of fainting.

Older people often have orthostatic hypotension – a drop in blood pressure when they rise from sitting or lying down. Their nervous system fails to constrict blood vessels as it should when there is such a position change. Consequently, their blood pressure drops, and they feel quite woozy.

Contracting your leg muscles many times in advance of standing keeps pressure up. Drinking two 8-ounce glasses of water helps people with orthostatic hypotension keep their pressure up for two hours after drinking the water. Increasing the amount of salt a person eats retains body fluid and also prevents a drop in pressure when rising.

Your surgery and subsequent weight loss might be the source of your blood-pressure problem. There are medicines that can keep pressure elevated, but why not try a very simple trick: Wear compressive stockings. They keep blood flowing back to the heart and prevent pressure drops. A snug abdominal binder is another way to keep blood flowing back to the heart.

If others who have had weight-reduction surgery have this problem, would they drop me a line?

DEAR DR. DONOHUE: What is the difference between hyperthyroidism and hyperparathyroidism? What is the treatment, and what kind of doctor provides such treatment? – Anon.

ANSWER: Hyperthyroidism is an overactive thyroid gland. That gland is located in the lower third of the neck. Its hormone keeps body cells and organs working at optimum speed. Thyroid hormone influences the way we metabolize food and how we burn calories. Too much thyroid hormone throws the body into overdrive. People lose weight in the face of a hearty appetite. Their hearts race even when at rest. They feel hot when others are chilled. Often they suffer from diarrhea.

Hyperparathyroidism is overactive parathyroid glands — four small glands located directly behind the thyroid gland. Their hormone regulates blood calcium level. Hyperparathyroidism raises blood calcium, and the symptoms are many. Muscles can become weak. High blood calcium leads to kidney stones. It also makes people thirsty, drives them to drink water compulsively and naturally increases trips to the bathroom. Bones weaken because calcium is drawn from them. Constipation is a common symptom.

The first doctor to see is the family doctor. He or she will determine if you have either condition. If the family doctor thinks a specialist is necessary, the next step is an endocrinologist. The thyroid and parathyroid glands are endocrine glands.

DEAR DR. DONOHUE: In a short time I will be 80 years old. My hair has yet to turn completely gray. I have some gray streaks but not many.

I am tired of telling people I don’t dye my hair. Can you explain why mine is still almost entirely black? Both my older sisters and younger brother have gray hair. – M.S.

ANSWER: Melanocytes are pigment-producing cells, and they are found in hair follicles as well as other places. With age, their numbers in hair follicles drop off, as does the production of melanin pigment. Hairs without melanin are gray. By age 50, on average, half the hairs on a person’s head have much less melanin pigment in them. But the process starts much earlier. Pigment cells start to disappear in the 20s and 30s.

I can’t give you a hereditary explanation for your raven locks. All I can do is congratulate you for being exceptional.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.