Fingernail or toenail abnormalities can tell you a lot about your health. They’re often a sign of a fungal nail infection or injury, but can sometimes indicate a more serious underlying disease.

Visit your GP if your nails have changed in colour, texture, shape or thickness and you don’t know why (you haven’t injured your nails or been biting them).

This guide gives the most likely reasons for the following nail problems:

• brittle or crumbly nails 
• discoloured nails 
• white nails 
• thickened, overgrown nails 
• loose nails 
• indented spoon-shaped nails 
• pitting or dents on the nails 
• grooves going across the width of the nails 
• unusually curved fingertips and nails 
• white lines running across the nails 
• dark stripes running down the nail 
• red or brown little streaks under the nails 
• a destroyed nail 
• infected nail fold (painful, red and swollen skin next to the nail)

Brittle or crumbly nails

Brittle nails are often just a sign of ageing or long-term exposure to water or chemicals such as detergents and nail polish. Nails can be strengthened by taking biotin (vitamin B7) supplements, by wearing gloves for all wet work and by frequently applying moisturising cream to the nails.

Sometimes, brittle or crumbly nails can be caused by:

• a fungal nail infection, which can be cleared by taking a course of antifungal tablets (this especially tends to be the cause for crumbly toenails)
• a skin condition called lichen planus, which can just affect the nails
• an underactive thyroid or overactive thyroid, where the thyroid gland either doesn’t produce enough hormones or produces too many
• nail psoriasis, a long-term skin condition that can cause the nails to become crumbly

A less common cause of crumbly nails is reactive arthritis, an unusual reaction of the immune system affecting the joints, muscles and other parts of the body following an infection. If you have a combination of symptoms affecting different parts of your body, your GP might consider this condition.

Discoloured nails

The most common causes of a yellow nail are fungal nail infections or nail psoriasis.

Yellow nails can also result from any of the following:

• frequent application of nail varnish
• lymphoedema, a long-term condition that causes swelling of the skin
• permanent damage to your airways caused by bronchiectasis (a long-term lung condition)
• sinusitis (inflammation of the lining of the sinuses)
• inflammation of the thyroid gland, found in the neck
• tuberculosis (TB), a bacterial infection affecting the lungs
• jaundice (yellowing of the skin) due to liver disease
• some drugs, such as mepacrine or carotene  
• chronic paronychia (infection of the nail fold)

Green-black nails can be caused by overgrowth of bacteria called pseudomonas, especially under loose nails. This can be treated by applying antibiotic eye drops underneath the nails or soaking the affected nails in an antiseptic solution or vinegar.

Grey nails can be caused by medication such as antimalarials or minocycline.

Brown nails can sometimes be caused by thyroid disease, pregnancy, malnutrition and frequent use of nail varnish.

Red or yellow drop under the nail

If the discolouration looks like a drop of oil under the nail plate or is the colour of salmon, you may have the skin condition psoriasis.

Half white, half brown nails

Fingernails that are half white and half brown (brown near the tips) can be a sign of kidney failure, where the kidneys stop working properly. The link is not fully understood, but one theory is that kidney failure causes chemical changes in the blood that encourage melanin (a skin pigment) to be released into the nail bed. It’s also possible that kidney failure causes an increase in the number of tiny blood vessels in the nail bed.

It is estimated that up to 40% of people with kidney failure have “half-and-half” fingernails.

Half-and-half nails have also been seen in some people with AIDS and can be seen following chemotherapy.

White nails

If most of the nail plate has turned white and it is not because the nail has become detached from the nail bed, it is likely to be either a fungal nail infection or a sign of decreased blood supply to the nail bed, known as “Terry’s nails”.

Terry’s nails are typically white with reddened or dark tips and can be a sign of a wide range of medical conditions, including the following:

• liver cirrhosis (scarring and damage to the liver) – about 80% of people with cirrhosis have Terry’s nails
• liver, kidney or heart failure
• diabetes
• iron-deficiency anaemia, which is a reduced number of red blood cells due to a lack of iron in the body
• chemotherapy
• an overactive thyroid (where the thyroid gland in your neck produces too many hormones)
• malnutrition 

Thickened, overgrown nails

A common cause of thickened nails is a fungal nail infection. This can also cause them to discolour and become crumbly (see above).

Other possible causes of thickened or overgrown nails are:

• psoriasis, a long-term condition that tends to also cause red, flaky patches of skin
• long-term pressure from shoes that are either too small or too narrow over the toes
• reactive arthritis, where the immune system attacks the joints, muscles and other parts of the body following an infection

Severely overgrown horn-like nails

Sometimes, the big toenails become so overgrown and thickened that they resemble claws and are almost impossible to cut with conventional nail clippers. This nail disorder is known as onychogryphosis (“ram’s horn nails”), and is seen in older people or as a response to long-term pressure on the nails. Regular chiropody can help, but sometimes the nails need to be removed by a podiatrist or doctor.

Loose nails

It’s normal for a toenail to come loose and fall off after an injury to the toe. However, if you haven’t injured your nail, a loose nail is often caused by over-manicuring the nails and cleaning under the nails with a sharp object.

Less commonly, it may be a sign of one of the following health conditions:

• a fungal nail infection
• psoriasis of the nail – psoriasis is a long-term condition that tends to also cause red, flaky patches of skin
• warts that cluster around the fingernail
• an overactive thyroid, which means the thyroid gland in the neck is producing too many hormones 
• sarcoidosis, a condition where small clumps of cells form in the organs and tissues of the body
• amyloidosis, where protein builds up in the organs
• a problem with the connective tissue fibres in the body that support the organs and body tissues
• poor circulation, for example caused by smoking or Raynaud’s disease (where the skin of the fingers turns white as a response to cold)
• an allergic reaction to medicine (usually to a type of antibiotic) or nail cosmetics

A loose nail should be cut back to where it is detached in order to allow the nail to become reattached as it grows. You should not clean your nails with anything other than a soft nailbrush.

Indented spoon-shaped nails (koilonychia)

If your fingernails curve inwards like spoons (known medically as koilonychia), you may have one of the following disorders:

• iron-deficiency anaemia, which is a reduced number of red blood cells due to a lack of iron in the body
• haemochromatosis, where the body contains too much iron
• Raynaud’s disease, a common condition that affects the blood supply to the fingers and toes, causing them to turn white 
• lupus erythematosus, an uncommon condition where the immune system attacks the body’s cells, tissues and organs

Pitting or dents on the nails

Pitting or small dents on the surface of your nails can be a sign of any of the below diseases:

• psoriasis, a long-term condition that tends to also cause red, flaky patches of skin (10%-50% of patients with psoriasis have pitted nails)
• eczema, a long-term skin condition
• reactive arthritis, where the immune system attacks the joints, muscles and other parts of the body following an infection
• alopecia areata, patches of hair loss that tend to come and go

Grooves across the fingernails (Beau’s lines)

Deep lines or grooves that go from left to right across the nail are known as Beau’s lines. They may be a sign of:

• an illness that started a few months ago
• chemotherapy
• a previous injury
• previous exposure to extremely cold temperatures, if you have Raynaud’s disease (a common condition affecting the blood supply to the fingers and toes)

Illness, injury or cold temperatures can interrupt nail growth and cause nail grooves to form at the base of the nails.

These grooves tend to only be noticed a few months later, when the nails have grown and the grooves have moved up the nails to become visible. It takes about four to six months for a fingernail to fully grow out, and 6-12 months for a toenail.

Unusually curved fingertips and nails

Clubbing of the fingertips means the tissue beneath the nails thickens and the fingertips become rounded and bulbous. The fingernails curve over the rounded fingertips.

Clubbing is thought to result from increased blood flow to the fingertips. It can run in families and be entirely harmless. However, if it suddenly develops, it may be a sign of one of many possible medical conditions, including:

• long-term lung disease or heart disease, such as lung cancer, bronchiectasis, or endocarditis
• inflammatory bowel disease, a long-term disease that causes inflammation of the lining of the gut
• stomach cancer or bowel cancer
• cirrhosis (scarring of the liver)
• polycythaemia, a condition where the blood is too thick 

White lines running across nails

White spots or streaks are normal and nothing to worry about, but parallel white lines that extend all the way across the nails, known as Muehrcke’s lines, are a sign of low levels of protein in the blood. This can be due to liver disease or malnutrition.

Dark stripes running down the nail

If you have dark skin, it’s fairly common to find dark stripes running down your nails. This occurs in more than 77% of black people over the age of 20, and in most cases is perfectly normal.

However, it is not something to ignore: dark stripes may sometimes be a form of skin cancer that affects the nail bed, called subungual melanoma. It’s therefore important that your doctor checks it to rule out melanoma.

Generally, subungual melanoma only affects one nail. Also, it will cause the stripe to change in appearance (for example, it may become wider or darker over time) and the pigmentation may also affect the surrounding skin (the nail fold).

Red or brown little streaks under the nails

If you have what look like red or brown little streaks underneath the nails, it’s likely these are splinter haemorrhages – lines of blood caused by tiny damaged blood vessels.

Just a few splinters under one nail are nothing to worry about and most likely due to injury of the nail. However, if many nails are affected, these splinters may be a sign of lupus erythematosus, psoriasis, an infection of the heart valves (endocarditis) or another underlying disease.

A destroyed nail

Nails can be destroyed by:

• injury, including nail biting
• skin diseases such as psoriasis or lichen planus
• overgrowth of the surrounding tissues, which is usually harmless (for example, due to a wart or verruca) but can sometimes be cancerous

If one of your nails becomes destroyed and you don’t remember injuring it, it’s important that you see your GP.

Painful, red and swollen nail fold (paronychia)

Paronychia is the name for inflammation of the nail fold, which is the skin and soft tissue that frames and supports the nail.

This is most often caused by infection, injury or irritation. It’s about three times more common in women than in men. Sometimes, there is an underlying skin condition such as eczema or psoriasis or another medical condition such as diabetes or HIV.

Paronychia can be acute, where it develops over a few hours, or chronic, where it lasts more than six weeks.

Acute paronychia

Acute infective paronychia usually starts after a minor injury to the nail fold, such as from nail biting, picking or manicures. The affected area is red, warm, tender and swollen. After a while pus may be seen, which can form around the nail and may lift the nail.

Acute paronychia is often caused by Staphylococcus aureus bacteria, although any number of germs can be involved. Treatment is with antibiotic creams or tablets. If there is a large amount of pus, it can help to have this surgically drained.

Sometimes, acute paronychia is caused by the cold sore virus, in which case it is known as herpetic whitlow.

Acute paronychia can completely clear in a few days with treatment, but if it is not treated or does not respond to treatment, it can become long-term (chronic).

Chronic paronychia

Chronic paronychia may start more gradually and be more difficult to get rid of. It is most common in people who often have their hands in water or chemicals, such as cleaners, bartenders, canteen staff or fishmongers.

It may start in one nail fold but can affect several fingers. The affected nail folds are swollen and may be red and sore from time to time, often after exposure to water. The nail plate gradually becomes thickened and ridged as it grows, and may become yellow or green and brittle.

The skin in chronic paronychia is often colonised by a mixture of yeasts and bacteria. These germs thrive on previously damaged skin, without necessarily starting the skin damage in the first place.

To settle chronic paronychia, both the skin damage and the colonisation with germs needs to be addressed.

It can take months for chronic paronychia to clear, and after that up to a year for nails to grow back to normal.

You may find that doing things such as keeping your hands dry and warm, frequently using emollient hand cream and avoiding biting and picking your nails can help. 

See your GP if the condition is severe. Depending on the type of paronychia, creams and/or tablets may be prescribed. Use these as directed.

You may be referred to a dermatologist for further investigation of the underlying skin disease or for contact allergy testing, and for treatment advice.