The surprising things your fingernails can tell you about your health

Are you listening to what your fingernails are telling you about your health? Yes, your eyes may be the window to the soul, but your fingernails can give you a glimpse of what's going on elsewhere in your body. Besides keeping your nails looking nice, you probably don't spend too much time thinking about your nails, but they can offer valuable clues to your wellbeing.

According to the Mayo Clinic, our nails are made of layers of a protein called keratin. They grow from the nail matrix, located underneath the cuticle. As new cells grow, the older cells become hard and tightly packed and are eventually pushed out toward the fingertip. Because fingernails grow so slowly, they can offer a peek into your body's recent past.

The Mayo Clinic noted that healthy nails are pink and smooth. Some variations, such as ridges that run the length of the nail, are harmless while "others can signal health concerns." Worried about your talons? These are the surprising things your fingernails can tell you about your health.

Yellow fingernails could be a sign of a fungal infection

If you have yellow spots under your nails or the whole nail has turned yellow, you may have a fungal infection. Known as onychomycosis, fungal nail infections can cause discoloration and thickening of the nails. They may also make them more brittle. Fungal infections are more common on toenails, but fingernails are not immune. The infection is caused by dermatophytes, a type of fungi that requires keratin in order to grow.

According to the Centers for Disease Control and Prevention (CDC), fungal nail infections can be hard to cure and usually require prescription oral antifungal medication. In very severe cases, a doctor may need to completely remove the affected nail. To prevent fungal infections, the CDC recommends keeping nails clean, dry, and well-trimmed. You should also avoid sharing nail clippers.

However, yellow nails aren't always a health concern. Most often, they're simply the result of the nail cosmetics we use. As Self explained, dark-colored polishes can stain nails yellow because the coloring agent in them reacts with the nail plate. The formaldehyde in nail polish can also react with the keratin in nails, turning them yellow and making them brittle.

Low oxygen levels can lead to blue fingernails

Because our nails are on our extremities, far away from our lungs and heart and fed only by small blood vessels, they can be an excellent indicator of low oxygen levels in the blood. Healthline noted that the nail bed turns purplish-blue when it doesn't get enough oxygen, and that color shows through the translucent nail above. Sometimes the lack of oxygen is only temporary and due to cold weather; the temperature causes the blood vessels in the fingertips to contract, preventing oxygen-rich blood from reaching the nail bed. But if your nails are blue all the time, this may indicate a more serious underlying health problem.

According to the Mayo Clinic, there are many conditions related to breathing or circulation that can cause low oxygen levels (hypoxemia). These include chronic obstructive pulmonary disease (COPD), emphysema, asthma, congenital heart disease, sleep apnea, and even certain medications.

A pulse oximeter can be used to measure blood oxygen levels. Normal values range between 95 and 100 percent, and anything under 90 percent is considered low.

Kidney disease can cause fingernail color changes

In a paper published in the Journal of General Internal Medicine in 2016, medical doctors Siwadon Pitukweerakul and Sree Pilla discussed a phenomenon known as Lindsay's nails. Also known as half-and-half nails, Lindsay's nails are white near the cuticle, but the 20 to 60 percent of the nail bed closest to the fingertip is a reddish-brown. Although it's unclear why this color change happens, it may be due to a higher concentration of beta-melanocyte-stimulating hormone. The paper's authors noted that Lindsay's nails, named after the researcher who first documented them in 1967, are found in approximately 40 percent of individuals with chronic kidney disease.

According to the Centers for Disease Control and Prevention, chronic kidney disease (CKD) occurs when your kidneys can't filter blood properly. This can cause waste products to build up in the body, which in turn can cause other health issues, including high blood pressure, heart disease, stroke, and early death. CKD is a progressive condition and can eventually lead to kidney failure. An estimated 37 million Americans — 15 percent of adults — are believed to have CKD, but 9 out of 10 are unaware they have the condition.

Liver disease can cause changes in fingernail coloration

In addition to Lindsay's nails, the 2016 paper published in the Journal of General Internal Medicine also made mention of a phenomenon known as Terry's nails. In this condition, approximately 80 percent of the nail bed is white, with a 0.5–3 mm brown or pink band at the end of the nail.

Healthline noted that the surface of Terry's nails may also change, appearing like ground glass. The whiteness of the nail is believed to be due to fewer blood vessels and more connective tissue than is seen in typical nails. The American Academy of Family Physicians reported that although Terry's nails aren't always associated with cirrhosis, 80 percent of individuals with this liver condition have this distinctive nail coloring.

According to the National Institutes of Health, cirrhosis occurs when the liver is permanently scarred. This scar tissue replaces healthy liver tissue and prevents the organ from functioning properly. This can lead to a number of complications, including infection, cancer, and insulin resistance. Eventually the scar tissue may lead to liver failure. Approximately one in 400 adults have cirrhosis, although lack of proper diagnosis may mean that the actual number is higher.

Low protein levels in your blood may show up in your fingernails

In addition to color changes, lines and grooves on the nails can be a sign that something potentially serious is going on with your health. According to WebMD, Muehrcke's lines are a double band of white lines that go across the nail and don't grow out as the nail grows. The lines do not cause dents in the nail, and the nail bed underneath the nail is healthy. These lines may temporarily disappear when pressure is applied. They're usually most clearly seen on the index, middle, and ring fingers and rarely visible on the thumb.

A 2018 paper published in the journal Autopsy & Case Reports explained that Muehrcke's lines, named after the researcher who first documented them in 1956, are associated with hypoalbuminemia (low albumin levels in the blood). Albumin is a protein found in blood plasma that helps maintain blood pressure and transports hormones and other substances throughout the body. Albumin levels below 3.4 g/dL are considered low. Causes of hypoalbuminemia include kidney damage, liver failure, too little protein intake, and inability to properly absorb protein. The paper noted that once albumin levels return to normal, Muehrcke's lines disappear.

A dark streak under your fingernail could be melanoma

Although fair-skinned individuals are 20 times more likely to be diagnosed with melanoma than darker-skinned individuals, acral lentiginous melanoma (ALM) is a subtype that's most commonly seen in people with darker skin or Asian ancestry. ALM is actually unrelated to sun exposure and often develops under a nail, where it creates a dark, narrow vertical stripe. The thumb or big toe are the digits most often affected. As the cancer progresses, the nail may crack and break.

According to a study published in the Archives of Dermatology in 2009, ALM has a lower survival rate than other types of melanoma. The study's authors suggested this may be because it's more likely to have reached a later stage before being diagnosed. The five-year survival rate for ALM was 80.3 percent (versus 91.3 percent for all melanomas), while the 10-year survival rate was 67.5 percent (versus 87.5 percent for all melanomas).

However, not all dark lines under the nail are dangerous. According to Healthline, black or reddish-brown vertical lines under the nail, known as splinter hemorrhages, are most often caused by trauma to the nail bed, leading to a burst blood vessel. Still, you'll want to consult your dermatologist to make sure.

Many illnesses can cause your fingernails to temporarily stop growing, leading to Beau's lines

Because your nails are continually growing, anything that temporarily disrupts that growth will create visible damage to the nail. In a 2015 paper published in the Indian Dermatology Online Journal, Dr. Archana Singal and Dr. Rahul Arora referenced Beau's lines, horizontal grooves in the nail that result from a brief pause in nail growth. If a groove only appears on one fingernail, it's most likely due to trauma to that particular nail matrix. But if Beau's lines appear on all fingernails and toenails, it's most likely a sign of a systemic illness or condition.

The authors noted that there are many things that may halt nail growth, including diseases that cause a high fever (such as mumps), pneumonia, an under-active parathyroid gland, and Kawasaki disease (inflammation of the blood vessels). The width of the groove indicates how long the nail was unable to grow, and if nail growth stops for two weeks or more, the Beau's lines may become so deep that they cause the entire nail to separate from the nail bed.

Psoriasis can affect your fingernails

No one ever wants to lose a nail, but how the nail separates from the nail bed is a good indicator of why it fell off in the first place. Onycholysis is separation of the nail beginning at the fingertip end and working back toward the cuticle. Trauma to the nail is usually the cause of this relatively common form of nail shedding. But when the nail separates beginning at the cuticle side (onychomadesis), psoriasis may be the cause.

Psoriasis is a common inflammatory condition in which an overactive immune system causes skin cells to regenerate faster than normal. According to the National Psoriasis Foundation, 30 percent of individuals with psoriasis also have psoriatic arthritis, in which the joints, tendons, and ligaments are affected by inflammation. A 2017 paper in the journal Reumatologia found that about half of individuals with psoriasis and 80 percent of people with psoriatic arthritis had associated nail issues. These issues can include pitting, changes in shape, thickening, and discoloration.

A 2017 paper published in the journal Cutis noted that once onychomadesis occurs, the nail or nails usually spontaneously begin to regrow within 12 weeks.

Thyroid disease can cause many fingernail changes

If you're having issues with your thyroid — the butterfly-shaped gland that sits in front of your windpipe and controls a wide variety of processes related to growth and metabolism — it can cause many issues throughout your body, including your nails. In fact, according to the American Academy of Dermatology, a dermatologist may be the first doctor to suspect you have a thyroid issue because many symptoms can be seen on the nails, skin, and hair.

Depending on whether your thyroid is over- or under-active, nails may grow slower or faster than usual. They may also become thick, dry, soft, or brittle and prone to breaking or crumbling. Nails may lift up or curve down around a fingertip that appears swollen with thick skin at the cuticle.

Thyroid disease is an umbrella term for any condition that causes your thyroid to produce too little or too much hormones. Thyroid disease is common, affecting approximately 20 million Americans. Women are five to eight times more likely than men to have thyroid disease. Thyroiditis (inflammation in the thyroid), insufficient or excessive iodine intake, Hashimoto's disease, and Graves' disease are the most common causes of thyroid disease.

Spoon-shaped fingernails are a sign you're not getting enough iron

Iron deficiency is extremely common. WebMD noted that 20 percent of women have low iron levels. When iron deficiency becomes severe, it can lead to anemia. Typical symptoms of anemia include extreme fatigue, weakness, and shortness of breath. But iron deficiency can also change the nails, causing them to become brittle. Anemia can also alter the shape of your nails.

Koilonychia, also known as spoon nails, is a reverse curvature of the nails, causing them to become concave. Iron deficiency is the most common cause of koilonychia, but other possible causes include autoimmune conditions such as lupus, celiac disease, and psoriasis as well as cardiovascular problems and vitamin B deficiency.

A 2017 paper published in the Journal of the American Academy of Dermatology noted that it isn't always easy for a doctor to recognize koilonychia because the concavity of the nails may be very slight. The paper's authors suggested using a simple "water-drop test" in which a drop of liquid is placed on the nail. If it remains in place rather than sliding off, this indicates that the nail is concave and an underlying condition like iron deficiency may be to blame.

Inflammatory bowel disease can cause finger and fingernail changes

According to the Centers for Disease Control and Prevention, there are two types of inflammatory bowel disease (IBD): Crohn's and ulcerative colitis. Although they affect different parts of the gastrointestinal tract in different ways, both are autoimmune conditions that cause inflammation, pain, diarrhea, fatigue, and weight loss. The Crohn's and Colitis Foundation estimated that 1.6 million Americans have IBD.

One surprising non-GI symptom of IBD is finger clubbing, a condition in which the tips of the fingers become wider and nails are abnormally shiny and curved downward around the enlarged fingertip. An investigation published in the British Medical Journal found that 38 percent of surveyed individuals with Crohn's had finger clubbing and abnormally curved nails while 15 percent of individuals with ulcerative colitis had the condition. The researchers noted that the severity of finger clubbing and nail curvature correlated to the severity of the individuals' IBD.

In a 2015 paper published in the Indian Dermatology Online Journal, Dr. Archana Singal and Dr. Rahul Arora noted that many other conditions can also cause finger clubbing, including certain types of congenital heart disease, cystic fibrosis, lung cancer, and infections of the heart and lungs.

Heavy metals and other toxic substances can affect your fingernails

According to the National Organization of Rare Disorders, inadvertent heavy-metal poisoning can occur either with a single high-dose exposure or, more commonly, through many low-level exposures that cause the metals to accumulate in the body over time. The metals most commonly associated with accidental poisoning are lead, mercury, arsenic, and cadmium. Even if you don't work in industries that use these substances, you may still be exposed through pollution, contaminated food (such as mercury in fish or cadmium in rice), drinking water from lead pipes or using cookware contaminated with lead, and accidental consumption of pesticides from unwashed produce.

Because most accidental poisoning happens gradually over time, nails may reveal clues that something's going on before other symptoms appear. An article published in the Dermatology Times in 2014 noted that Mees' lines on the nails can be a sign of heavy-metal poisoning. Also known as leukonychia striata, these bands of discoloration run horizontally across the nail. The article noted, however, that these lines may also appear because of a nail injury and in patients with kidney failure or undergoing chemotherapy.

Are your fingernails trying to tell you you're allergic to your manicure?

Many of us enjoy getting a mani-pedi as a way to treat ourselves, but some people experience allergic contact dermatitis when they come into contact with ingredients commonly found in nail cosmetics. The most common offenders are tosylamide formaldehyde resin, a preservative commonly found in base coats, where it helps the polish adhere to the nail, and butyl acetate, a solvent used in polish remover.

Allergic reactions on fingers and nails include red, swollen skin, discolored nails, and white spots on nails. Other areas that people touch often, such as the eyelids and the back of the neck, can also react when they come into contact with the offending nail product. Baylor College of Medicine noted that people can also be allergic to chemicals known as acrylates found in acrylic, shellac, and gel nails. Allergic reactions include redness, flaking, and itching around the nail but may also cause a rash on the face.

Unfortunately, imply switching to a hypoallergenic nail polish may not be enough. A 2017 paper found that even though many brands of hypoallergenic polishes don't contain tosylamide formaldehyde resin,they still contain at least one other ingredient capable of triggering an allergic reaction.

Constantly biting your fingernails? It may be a sign of anxiety

If you bit your nails as a kid, you're not alone. The University of Michigan noted that nail biting is the most common nervous habit and approximately half of all children between the ages of 10 and 18 regularly bite their nails. Some young adults continue to do so, but most people stop biting their nails by around age 30. But if you have a constant, uncontrollable urge to bite your nails, you may have what's known as onychophagia.

Along with other "body-focused repetitive behaviors" like hair pulling and skin picking, out-of-control nail biting is considered a type of obsessive-compulsive disorder. People with the condition often feel stress or anxiety before biting, relief from these feelings during and immediately after biting, and then shame or embarrassment at the damage such biting can cause. Nails and cuticles can become torn, ragged, and irritated, and the constant biting may also lead to dental problems and infections in the mouth or around the nail.

Treating compulsive nail-biting often requires some form of psychotherapy, such as cognitive behavioral therapy (CBT) or acceptance and commitment therapy (ACT), as well as physical barriers like gloves to prevent biting.