It's a little butterfly-shaped dictator: The thyroid gland, located in the neck, weighs less than an ounce but affects the way all your body's cells function. If it's overactive (secretes too much of a hormone), you might lose weight, feel nervous or jittery and have trouble sleeping. If it's underactive, you might gain weight and feel tired and sluggish.

Researchers at the Chelsea and Westminster Hospital in London recently found that HIV-positive people have slightly higher rates of thyroid disturbance than negative people, partly as a result of med side effects. Whatever the cause, thyroid problems respond well to treatment. But first you need your thyroid function monitored—a simple blood test does the trick. Put it on the list for your next doctor's appointment. (For more information, search “thyroid” at