Your nails can reveal clues about your overall health, but changes in their appearance aren't always cause for concern. Things like yellowing, ridges, or brittleness are often related to aging or everyday wear and tear. In some cases, however, they may be linked to underlying health conditions.

1. Yellow Nails

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The appearance of yellow nails can happen naturally as you get older, but aging is not the only cause. Other possible factors include:

• Nail fungus (onychomycosis): This is the most common medical nail condition, accounting for about half of all nail disorders doctors diagnose. It usually starts as a white, yellow, or yellow-brown discoloration before the nail becomes thicker, brittle, or crumbly. While it can affect fingernails, it's much more common in the toenails.1
• Nail polish staining: Both nail lacquers and acrylic nails can lead to discoloration with overuse, John Anthony, MD, a dermatologist at the Cleveland Clinic in Ohio, told Health.
• Smoking: This can stain nails and give them a yellowish hue.
• Psoriasis: Nail changes are common among people with psoriasis, and can include yellow-brown discoloration ("oil drop" spots), pitting, and thickening.2
• Yellow nail syndrome. This extremely rare disorder is thought to affect fewer than 1 in a million people. It causes thick, slow-growing yellow nails that usually occur with respiratory issues and limb swelling.3

2. Dry, Cracked, or Brittle Nails

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About 20% of people experience brittle nails,4 and the reasons why run the gamut.

Debra Jaliman, MD, a New York-based dermatologist and author of "Skin Rules," attributes them to things like:

• Swimming in chlorinated pools
• Overuse of nail polish remover
• Frequent dishwashing without gloves
• Living in a low-humidity environment

Speak with a healthcare provider if brittle nails are an ongoing, unexplained problem. Conditions like psoriasis, eczema, and hypothyroidism also cause brittle nails.56

3. White Spots

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White spots on nails, called leukonychia, are extremely common and almost always harmless.7 While there's a commonly-held notion that white spots are indicative of a calcium deficiency, that's not the case; they're usually the result of minor trauma or bumps to the area where your nail grows.7

4. Horizontal Ridges (Beau's Lines)

Deep, horizontal ridges or grooves, also known as Beau's lines, happen when nail growth is temporarily interrupted. There are several reasons this can occur:8

• Infection or illness: If your body is working overtime against something like fever, flu, pneumonia, or even measles, your nails might temporarily stop growing. Because they nails resume growing once you're recovered, Beau's lines are indicative of past illness; if you see them, it doesn't mean you're still sick.
• Raynaud's disease: During a Raynaud's episode, reduced blood flow to the fingertips can temporarily disrupt the nail matrix—the tissue responsible for producing new nail.
• Medication, especially chemotherapy: About half of people undergoing chemotherapy experience Beau's lines.9

5. Pitting

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You might have nail pitting if you notice small, shallow dents on the surface of your nails.10

The cause is usually a type of inflammatory skin condition, including:

• Psoriatic arthritis (about 80% of people with psoriatic arthritis experience nail changes11)
• Psoriasis
• Eczema
• Alopecia areata

6. Clubbing

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Reproduced with permission from © DermNet and © Raimo Suhonen dermnetnz.org.

Nail clubbing is much less common than issues like yellowing nails or pitting.

"Clubbing of the nails is when the ends of your fingers swell and the nail becomes curved and rounded," Anthony said. The nail becomes convex and curves downward over the fingertip, creating a bulbous appearance.

When it develops, clubbing is usually associated with an underlying medical condition—80% of cases are associated with diseases of the lung and chest area, like:12

• Lung cancer
• Bronchiectasis
• Cystic fibrosis

Another 5% are linked to gastrointestinal conditions such as inflammatory bowel disease (IBD).12

Contact a healthcare provider as soon as possible to determine the cause of this nail condition. While it may be a harmless genetic trait you inherited, an underlying cause is more likely.

7. Spoon Nails

"Spoon" nails, also called koilonychia, are nails that become unusually thin and curve inward, creating a shallow depression that can sometimes hold a drop of water.12 While this nail shape can occasionally result from repeated trauma or certain skin conditions, it's most often linked to an underlying health issue.

"The most common cause is iron deficiency anemia," said Jaliman.

Less commonly, spoon nails may be associated with conditions such as hypothyroidism or autoimmune diseases.13 If you notice your nails becoming spoon-shaped and don't know why, it's a good idea to see a healthcare provider, who can check for iron deficiency and other possible causes.

8. Dark Stripes or a Painful Growth

Most dark streaks or black discoloration under a nail aren't serious. A bruise from stubbing your toe or slamming your finger in a door (called a subungual hematoma) is a common cause and will usually grow out with the nail over time.

However, you should see a dermatologist if you notice a new dark brown or black stripe that doesn't appear to be related to an injury, especially if it affects just one nail, gets wider or darker over time, extends onto the surrounding skin, or is paired with a persistent, painful growth. These changes can be a sign of subungual melanoma, a type of skin cancer.14

American Academy of Dermatology Association. How to check your nails for melanoma.