By Helen Fields

Women tend to get all hung up on their skin's cosmetic attributes. Is that a new wrinkle? How can I make my elbows smoother? Will this self-tanner turn me orange? But their concern with skin as upholstery ignores a larger truth: Skin is a big organ that you can see. Because the skin is connected to the rest of the machinery--blood vessels feed it, nerves tell it what to do--diseases that affect the systems of the body often give an early warning on the skin. "You pick the disease, and I'll tell you the skin manifestation," says Bob Brodell, a dermatologist in Warren, Ohio. He remembers seeing an 8-year-old girl with hives when he was a young resident in the emergency room who a few weeks later was diagnosed with leukemia--which has been known to trigger hives.
Indeed, doctors--especially dermatologists like Brodell--keep their eyes on the skin for all manner of clues to underlying disease and other conditions affecting a woman's health. If you have an infected heart valve, bits of the infection can break off, get stuck in tiny vessels, and show up as streaks under your fingernails. Pregnancy makes some women itch all over; anemia can cause the hair to fall out. One morning in 1988, when Birmingham, Ala., nurse Susan McNaughton and her husband, a doctor, were standing at the double sinks in their bathroom, he looked at her in the mirror and announced, "You have lupus." The clue: the disease's characteristic butterfly rash, spreading light pink across the top of her cheeks and the bridge of her nose.

Heads up. Rashes and hives and dark spots under the nails are usually not a sign of serious illness, of course. That butterfly-shaped discoloration might just be acne; you can get marks under your nails during an energetic round of gardening or housework. But combined with other signs such as weight loss or fatigue, symptoms on the skin can give doctors a hint of what is going on below. McNaughton's illness, systemic lupus erythematosus, is one of a group--autoimmune diseases--that stands out for affecting women in particular and showing up on their skin. These diseases happen when the body's attack mechanism goes awry: Your immune system mistakes your own tissues for foreign invaders and turns its wrath on them.

And often, the skin takes a beating. In most people with lupus, the self-attack inflames joints, causing arthritis. The liver and kidney can become inflamed, too, and stop working as well as they should. Meantime, the proteins that cause inflammation travel through the blood vessels to the skin, where they can cause rashes like McNaughton's. Her skin has become so sensitive to sunlight that she now leaves her apartment in the daytime only for doctors' appointments.

Likewise, one of the first signs of the autoimmune disorder scleroderma is tight, thick skin. In most people, the thickening stops at the skin. But the systemic form of scleroderma can cause serious harm below the surface, when the tightening proceeds in internal organs and essentially replaces vital tissues with scars. In the lungs, for example, the thickening can impede the exchange of oxygen and carbon dioxide. Many people develop gastrointestinal woes.