Under the polish is a health report. A Manhattan podiatrist reads the signs, from the common to the one you should never ignore.

The short version: Yellow, thick toenails usually mean a fungal infection. Horizontal grooves, spoon-shaped nails and other changes can reflect illness, injury or nutrition. A dark streak in a nail can be a sign of melanoma and should be examined promptly. Manhattan podiatrist Dr. Rebecca Pruthi recommends regular breaks from polish so changes can be seen.

There is a certain kind of New York woman whose toenails have not been seen in their natural state since iPhones had a home button. Gel in winter, a bright red for the Hamptons, a nude for the office, a deep plum for fall. The color changes with the season. The nail underneath never gets a day off.

Dr. Rebecca Pruthi, who owns Foot Care of Manhattan on Madison Avenue, has argued for giving it one. In a column on the Dr. Oz blog she wrote that our natural nail condition can offer valuable clues to our underlying health. Medical problems don’t usually announce themselves first in the nails, she noted, but unusual growth or color can point to a condition the rest of the body hasn’t revealed yet.

In other words, your toenails are a slow-moving health report. Polish is a cover page nobody ever turns.

Why toenails tell the truth slowly

Toenails grow slowly, much more slowly than fingernails. That slowness makes them useful as a record. A stubbed toe, a high fever or a stretch of poor nutrition can leave a mark that grows out over many months, like a tree ring for your last bad winter. It also means changes appear gradually and are easy to miss, especially under three coats of lacquer.

Yellow and thick: usually fungus

Yellowing and thickening is the most common nail problem Pruthi sees. It usually signals a fungal infection, and as it progresses the nail can darken, turn brittle and press painfully on the nail bed. Less often, she noted, yellow nails can go along with thyroid conditions, diabetes, psoriasis or lung problems.

Fungus is also sneaky on the skin. Pruthi told Women’s Health that many patients with peeling soles assume they are just dry, when the cause is a fungal infection that doesn’t itch. Left alone, it plays tennis between foot and nail: “the skin infects the nails, the nails re-infect the skin.” Her advice is to treat whichever shows up first.

Treatment takes patience, because the healthy nail has to grow out to replace the infected one. Options include topical and oral medications, and her practice lists laser toenail treatment among its services. Results vary from person to person, so ask your podiatrist which approach fits your nails and what to expect along the way.

Lines across the nail

Horizontal grooves, called Beau’s lines, mark the place where nail growth paused. Pruthi listed trauma, illness, poor nutrition and chemotherapy as common causes, along with diabetes, high fevers and circulation disease. A stubbed toe at a wedding can do it. So can a bad flu. The line simply grows out as a record of the interruption.

One groove on one nail usually points to a local injury. Grooves across several nails at the same level suggest something that affected the whole body at once, which is worth mentioning to your doctor.

Spoon-shaped nails

Nails that thin out and scoop upward like a spoon can follow iron deficiency anemia, liver problems, heart disease or an underactive thyroid, Pruthi wrote. This is one a pedicurist will often polish right over, and one worth a conversation with your physician.

Black, but not dangerous

Not every dark nail is alarming. Shoes that are too short or too tight, long walks downhill, or a toe that hits the front of a pump all day can bruise the nail bed and turn the nail black or purple. Runners and heel devotees know this look well. A bruise usually traces back to a specific event and moves forward as the nail grows.

White, chalky patches that appear after you remove polish are often superficial changes from the polish and remover themselves, another argument for a regular break.

The dark streak: don’t wait

The sign that matters most is a dark streak running down the length of the nail, or dark pigment spreading near the cuticle. It can indicate melanoma, a serious skin cancer where early detection is key. “I always mention melanoma because it can be so easily overlooked,” Pruthi wrote. Feet are the part of the body we check least and cover most.

The practical rule is simple. A dark line that you can’t connect to an injury, that doesn’t grow out, or that widens or darkens should be examined by a doctor promptly. Most turn out to be harmless. The few that aren’t are far easier to treat when they’re found early.

A composite story

Call her Celeste, a composite: 52, an Upper West Side philanthropist who sits on two museum boards and has worn gel polish nearly year-round for a decade. Her calendar runs on galas, and so does her pedicure schedule. Before a September trip she let a new technician soak the color off, and there it was, a thin brown line down her big toenail.

Her pedicurist said it was probably a bruise. Celeste couldn’t remember stubbing anything. She had read about this sort of thing, and booked a podiatrist that week instead.

The polish break

Pruthi’s practical prescription is simple. Between salon visits, take a breather and let the nails go without polish. While you’re on the beach, take a quick look at your toenails. Any change in color, shape or growth, have it examined. And while you’re at it, she noted to Women’s Health, put sunscreen on your feet, the most forgotten skin at any beach club.

A few habits keep nails healthier between those check-ins:

  • Cut nails straight across, not curved, to prevent ingrown corners.
  • Keep feet dry, change damp socks, and rotate shoes so they dry out between wearings.
  • Wear sandals in gym showers, pool decks and hotel bathrooms.
  • Don’t share clippers, files or towels, even with family.
  • Choose shoes with toe room, so nails aren’t jammed into the front of the shoe all day.

Celeste’s September

The brown line turned out to be harmless. Celeste got the answer in days instead of months, flew to Italy with a clear mind, and came home with a new rule. She now keeps one bare-nail week a month, the way some women keep a dry January. She calls it her board meeting with her feet. Attendance is mandatory.

Frequently asked questions

What does a dark line on a toenail mean? A dark streak can come from a bruise, a mole in the nail matrix or, rarely, melanoma. A line you can’t connect to an injury, that doesn’t grow out, or that widens or darkens should be examined by a doctor promptly.

Why are my toenails yellow and thick? The most common cause is a fungal infection. Less often, yellow nails can be associated with conditions such as psoriasis, diabetes, thyroid problems or lung disease. A podiatrist can confirm the cause.

Is toenail fungus contagious? Yes. Nail and skin fungus can spread between your own feet and nails, and to others through shared tools, towels, shoes and damp floors. Treating skin and nails together helps prevent reinfection.

Does laser treatment work for toenail fungus? Laser treatment is one option alongside topical and oral medicines. Results vary from person to person, and improvement takes time because the healthy nail must grow out. Ask your podiatrist what to expect for your nails.

Back to the series pillar: Manhattan’s Most Glamorous Women Walk Miles in Heels. Related: Your Pedicure Looks Clean. Your Footbath Might Not Be.

Celeste is a composite. Any dark streak or spot on a nail should be examined by a doctor promptly. This article is for general information, not medical advice.