It was a miserable year for Carol, 55, of St. Louis. She had the classic sign of a yeast infection — vaginal itching that was driving her crazy. But the OTC antifungal cream she bought gave her only temporary relief; her symptoms returned with a vengeance and lasted for weeks. "I felt I was never going to get over it," she says. Finally, she went to a specialist at a vaginal disease clinic, who spent most of the visit asking questions — then suggested that Carol switch to a gentler soap and laundry detergent. Within 6 weeks, her symptoms were gone for good.

Whether you have an itch or an ache, it's easier than ever these days to take a do-it-yourself approach to fixing the problem. Lots of drugs that once required a prescription are now available over the counter, including Carol's antifungal medication. Add to that the accessibility of instant health information on the Internet — 80 percent of American Internet users have done health-related searches, according to the Pew Internet and American Life Project — and we've become a nation of self-healers. That means people are taking charge of their health care, says Nimita Thekkepat, PharmD, an assistant professor of harmacy practice at St. Louis College of Pharmacy. But, she warns, self-help can also mean mistakes.

Here are four common conditions that people often misdiagnose — and the clues that can lead to real relief.

You think you have ...  a yeast infection.

It may be... irritation from soap or other personal products, menopausal dryness, or an STD.

Nearly 75 percent of women who visited a vaginal disease clinic thinking they had a recurrent yeast infection actually had a different issue, according to recent research. Most had self-treated for months or years, says study author Susan Hoffstetter, Ph.D., an assistant professor in the department of obstetrics and gynecology at St. Louis University School of Medicine. In 70 percent of cases the cause of their discomfort was irritation from soap, menstrual pads, or other personal products. Other women had an overgrowth of normal vaginal bacteria; a few had the sexually transmitted disease chlamydia.

Using an unnecessary antifungal is fruitless, Hoffstetter says, and can set you up for more irritation by upsetting the balance of vaginal microbes. And your untreated bacterial infection can be dangerous: Any chronic vaginal infection raises your risk of catching a sexually transmitted disease, including chlamydia, which can cause inflammation in the fallopian tubes and abdomen.

Treat it right. Before you use an anti-fungal, ask yourself whether the irritation started after you switched soap or other personal products; if so, try going back to your old routine. Irritants include antibacterial soaps, perfumed wipes, and moisture-trapping
synthetic underwear. For menopausal dryness, vegetable-based oils in the area (Hoffstetter recommends olive oil) may help, or ask your doctor about using an estrogen cream, vaginal tablet, or vaginal ring. If you try an antifungal, it should offer relief within 2 days. If it doesn't — or if symptoms recur within 2 months — ask your gynecologist to determine whether you have a bacterial infection instead.

You think you have ... depression.

It may be... hypothyroidism or sleep apnea.

As the stigma of depression has eased, the number of people diagnosed with it has increased. That's good — but self-diagnosis isn't smart, experts say. One reason: Some symptoms of mild to moderate depression — low energy, sleepiness, and a down mood — are similar to those of thyroid disease, says Jennifer Davis, PhD, an assistant professor of psychiatry at the Warren Alpert Medical School of Brown University. Nearly 10 percent of Americans are thought to have undetected hypothyroidism — an underperforming thyroid gland.

Sleep apnea can also cause depression-like symptoms. Some 80 percent to 90 percent of adults with this condition go undiagnosed, according to the American Academy of Sleep Medicine. Missing either diagnosis can mean trouble: Hypothyroidism can cause high cholesterol and heart disease. And sleep apnea raises your risk of diabetes, hypertension, stroke, and heart attack.

Treat it right. The blues, along with fatigue, weight gain, dry skin, hair loss, or intolerance to cold, may mean a slow thyroid. A blood test can measure thyroid hormone levels; treatment is a daily dose of synthetic thyroid hormone. Sleep apnea signs are snoring and daytime sleepiness. The best test: a visit to a sleep lab. Lifestyle changes or wearing a special sleep mask may help.

You think you have ... a sinus headache.

It may be ... a migraine.

In a recent study of 100 headache sufferers who believed they had sinus headaches, 89 percent actually had some form of migraine, says study author Eric Eross, DO, director of Scottsdale Headache Center at Arizona Neurological Institute. Less than 50 percent of Americans with migraines get a correct diagnosis, experts say — and sinus headache tops the list of mistakes.

If you've wrongly pegged your headache, you're likely getting little relief. OTC sinus or pain drugs tend to be less effective for migraines than are prescription triptans, says Brian M. Grosberg, MD, director of the Inpatient Headache Program at Montefiore Headache Center in the Bronx. This may lead to excessive use of pain relievers, rebound headaches, and even needless CAT scans or sinus surgery.

Treat it right. It's easy to misdiagnose a migraine. Like a sinus headache, it can be triggered by either weather or allergens. Both types can cause facial pain, runny nose, and watery eyes, according to Eross. But it's probably a migraine if your headache has at least two of these four traits: It's one-sided, it throbs, the pain is moderate to severe, and it gets worse with physical activity.

A migraine may also cause nausea or sensitivity to light or sound, says Grosberg. (A sinus infection typically includes thick green mucus, fever, or bad breath.) Try to identify and avoid your migraine triggers; if you can't, ask your doctor about triptans and other relevant drugs.

You think you have... a urinary tract infection.

It may be ... interstitial cystitis.

By age 24, one-third of women have experienced a UTI. So when a woman has bladder pain, urgency, and a constant need to go, she often blames a bladder infection. Doctors may even make that same leap, says J. Quentin Clemens, MD, director of the division of neurourology and pelvic reconstructive surgery at University of Michigan Medical Center. Result: Patients with interstitial cystitis, which has similar symptoms and is also known as painful bladder syndrome, can go undiagnosed for years. Often, treatment of IC only partly relieves pain. Still, a diagnosis is a comfort, says Clemens — and means you avoid unnecessary antibiotics.

Treat it right. If you don't get relief after a standard (3- to 7-day) course of antibiotics, your doctor needs to do a urine test to confirm that you have a bacterial infection. If the test comes up negative, you may need to see a urologist for an IC evaluation.