- 01Fingernails only, or all twenty nails
Brittle fingernails with normal toenails usually mean an outside cause: water, soap, sanitizer, acetone and gel removal wear down the hands, while the feet are spared 1. When toenails turn brittle too, or the nails come with tiredness, hair shedding or feeling cold, the cause is more likely inside the body, and a blood test is worth doing. The exception is thick, yellow, crumbly toenails, which more often mean a fungal infection.
Most brittle nails are worn out from outside, by water, soap and solvents. When the cause is inside the body, the one to check first is low iron, then an underactive thyroid or too little protein 3. About one adult in five has nails that break easily, and women are affected roughly twice as often as men 1. Your toenails usually tell you which kind you have.
When to get help now
Most brittle nails are harmless, but some nail changes and the symptoms that come with them need a doctor.
- Chest pain: get emergency care now, and mention it if you take biotin.
- A new dark or brown streak running down a nail, especially a single nail or one that widens or stains the skin.
- A nail lifting away from its bed.
- Pitting in the nails with scaly skin patches.
- Fingertips that swell and curve over the nail (clubbing).
- Deep horizontal grooves across the nails after an illness (Beau's lines).
- Brittle nails with breathlessness or unexplained weight change.
- Heavy or prolonged bleeding, black stools, or bleeding after menopause.
LifeFrom can't examine you or assess a symptom. If something feels seriously wrong, trust that and get care rather than waiting on a test.
What to do next
- If only your fingernails split, try gloves and moisturizer for 3 months and watch the new nail at the base.
- If your toenails are brittle too, or you are tired, cold or losing hair, ask your doctor for ferritin, a complete blood count (CBC) and TSH.
- If you take biotin gummies, tell whoever orders the test before your blood is drawn.
- If ferritin is low, ask why it is low before taking iron, and ask when to recheck ferritin and hemoglobin once treatment starts.
What brittle nails look like, and what each pattern points to
Brittle nails come in a few distinct shapes, and the shape is a clue. Nails that peel in thin layers at the tip, like pastry, have what doctors call onychoschizia. Fine lengthwise ridges that crack from the tip back toward the cuticle are called onychorrhexis. Thin nails that dip in the middle and curl up at the edges into a shallow spoon are called koilonychia, and spoon nails are the classic sign of iron deficiency 3.
| Pattern | What you see | Most likely cause | Test worth doing |
|---|---|---|---|
| Layered peeling at the tip (onychoschizia) | Tip splits into thin sheets, often worse after washing up | Repeated wetting and drying, acetone, gel or acrylic removal | Usually none if toenails are normal |
| Lengthwise ridges and splits (onychorrhexis) | Fine lines from cuticle to tip, cracks along them | Age, dryness; sometimes low iron or thyroid | Ferritin and TSH if all nails are affected |
| Spoon-shaped, thin nails (koilonychia) | Nail dips in the middle and curls up at the edges | Iron deficiency | Ferritin and complete blood count |
| Dry, slow-growing, brittle nails everywhere | Nails barely need cutting; skin and hair dry too | Underactive thyroid | TSH, then free T4 |
| Brittle nails after rapid weight loss | Nails thin out over months | Too little protein or calories | Albumin and total protein, with a doctor's review |
| Thick, yellow, crumbly nails, often toenails | Nail lifts, discolors, flakes underneath | Fungal infection (onychomycosis), sometimes psoriasis | A nail clipping test for fungus, not a blood test |
Outside or inside? A quick check before any test
The fingers-versus-toes clue sorts most cases. Water makes the nail plate swell and drying makes it shrink, and van de Kerkhof and colleagues name those repeated cycles, along with detergents and solvents, as the main outside causes of brittle nails 1. Hands get that treatment from dishwashing, hand sanitizer, cleaning products, hairdressing, nursing and polish remover; feet mostly escape it. So brittle fingernails with healthy toenails point outward. Brittle toenails as well make an inside cause more likely, though thick, discolored or crumbly toenails are more often a fungal infection.
| Signs the cause is outside | Signs worth a blood test |
|---|---|
| Only fingernails split; toenails are fine | Toenails are brittle too, without thickening or yellowing |
| Wet work, frequent hand washing or sanitizer | Tiredness, breathlessness on stairs, pale skin |
| Regular acetone, gel or acrylic removal | Hair shedding, feeling cold, dry skin, weight gain |
| Over 60, with dry skin generally | Heavy periods, pregnancy, or a vegan or very restrictive diet |
| Nails improve on holiday or with gloves | Spoon-shaped nails or a recent rapid weight loss |
Brittle nails causes: which deficiencies matter, in order
Common causes of brittle nails
Outside causes are the most common; when the cause is inside the body, low iron comes first.
- 01Repeated wetting and drying
Water makes the nail plate swell and drying makes it shrink, and detergents, hand sanitizer and cleaning products add to it 1. Fingernails split while toenails stay normal.
- 02Polish remover, gels and acrylics
Regular acetone, gel or acrylic removal dries the nail and makes the tip peel in thin layers 1.
- 03Age and dry skin
Over 60, nails thin and dry with age, and lengthwise ridges become common without any deficiency.
- 04Iron deficiency
Low iron is the inside cause to check first, and spoon-shaped nails are its classic sign. Ferritin falls well before hemoglobin does 34.
Can be linked with Iron deficiency
- Ferritin
- Low iron stores, often before anemia appears
- Hemoglobin
- Whether low iron has gone on to cause anemia
- Transferrin saturation
- How much iron is available to use right now
- 05Fungal infection
Thick, yellow, crumbly nails, often toenails, are more often a fungal infection than a deficiency. A nail clipping test checks for it, not a blood test.
- 06Underactive thyroid
Nails grow slowly and turn dry and brittle, usually alongside tiredness, feeling cold and dry skin 13.
Can be linked with Hypothyroidism
- Thyroid Stimulating Hormone
- Rises when the thyroid is slow
- Free Thyroxine
- Confirms whether a high TSH means the thyroid is underperforming
- 07Too little protein or fast weight loss
Nails are made of keratin, a protein, so restrictive diets, weight-loss injections or weight-loss surgery can leave the nail plate thin 3.
- Albumin
- Whether protein intake or absorption is too low
- Total protein
- Whether protein intake or absorption is too low
- 08Zinc, B12 or folate deficiency
These can affect nails too, but the signs are less specific and usually travel with other symptoms, such as tingling hands and feet.
Can be linked with Zinc deficiency
- Zinc (Serum/Plasma)
- A zinc gap from diet or absorption
- Vitamin B12
- A B12 gap from diet or absorption
- 09Biotin deficiency
An ordinary varied diet supplies enough biotin, so deficiency is rare. The people at risk are heavy raw egg white eaters, people on some anti-seizure medicines and those with a rare inherited enzyme problem 7.
Brittle fingernails with healthy toenails point to an outside cause; brittle toenails as well make an inside cause more likely.
Of the deficiencies, low iron is the one most worth checking first. Iron deficiency is linked to brittle nails as well as spoon nails, and it shows up in ferritin (the protein that stores iron) well before hemoglobin falls 34. As our iron deficiency page puts it, "a normal CBC does not rule iron deficiency out." When low iron goes on long enough, it becomes anemia.
The thyroid comes next. Our hypothyroidism page describes it as a gland that "makes too little thyroid hormone, which slows the body down", and nails slow down with it: they grow slowly and turn dry and brittle, usually alongside tiredness, feeling cold and dry skin 13. It shows on a TSH test (thyroid-stimulating hormone, the pituitary signal that rises when the thyroid is slow), followed by free T4 (the thyroid hormone itself). The most common cause is Hashimoto's disease, an autoimmune attack on the thyroid.
Nails are made of keratin, a protein, and poor nutrition shows in the nails 3, so restrictive diets and fast weight loss, including on weight-loss injections such as semaglutide or tirzepatide, can leave the nail plate thin. If brittle nails follow an eating disorder, weight-loss surgery or unplanned weight loss, see a doctor rather than adjusting the diet alone. Zinc deficiency and low vitamin B12 or folate can affect nails too, but those signs are less specific and usually travel with other symptoms.
Biotin deficiency is the least likely cause. Adults need about 30 mcg of biotin a day (the official "adequate intake"), and an ordinary varied diet supplies it, so deficiency is rare 7. The people at real risk are a small group: heavy long-term raw egg white eaters, people on some anti-seizure medicines, and those with a rare inherited enzyme problem 7.
The markers that check the inside causes
The blood tests that sort brittle nails out
A short panel answers the inside question better than a long shopping list. Start with iron and the thyroid, and add the rest only when your story fits. Vitamin D, HbA1c and a broad metabolic panel answer other questions; they are worth adding when your symptoms point to diabetes, the kidneys or the liver, not for nails alone.
| Test | What it answers | When to add it |
|---|---|---|
| Ferritin | Are iron stores low? | First, for anyone with brittle nails and an inside sign |
| Complete blood count, or CBC (hemoglobin, and MCV: the average red cell size) | Has low iron caused anemia, with small red cells? | Alongside ferritin |
| Transferrin saturation | How much iron is available to use right now | If ferritin is borderline or you have inflammation |
| TSH, then free T4 | Is the thyroid underactive? | Second, especially with tiredness, cold or dry skin |
| tTG-IgA, with total IgA | Could celiac disease be blocking iron absorption? | If iron is low with no clear reason; only accurate while you still eat gluten |
| Albumin, total protein | Is protein intake or absorption too low? | After rapid weight loss or a restrictive diet |
| Zinc, vitamin B12 | Specific gaps from diet or absorption | Vegan diet, gut symptoms, or tingling hands and feet |
Our CBC guide walks through each line of the blood count, and our guide to iron studies shows how ferritin, iron and transferrin saturation fit together.
What counts as low ferritin depends on who sets the line. Ferritin is reported in µg/L (micrograms per liter), which is the same number as ng/mL, the unit many US labs print. The World Health Organization defines iron deficiency in healthy adults as a ferritin below 15 µg/L, and below 70 µg/L when infection or inflammation is present, because inflammation pushes ferritin up 4. The American Society of Hematology's 2026 guideline sets the line higher: 30 or less in adults, and 50 or less for people with symptoms, heavy periods or other risk factors 8. So a result between 15 and 30 reads two ways depending on the guideline.
Example ferritin result for someone with brittle nailsµg/L
If you already have iron results, enter them below to see how ferritin and transferrin saturation read together.
Where do your iron stores sit?
Enter your numbers. Nothing leaves your browser.
If iron is low and there is no clear reason, such as heavy periods or a vegan diet, celiac disease is worth testing for. The UK's NICE guideline NG20 advises offering celiac blood tests to people with unexplained iron-deficiency anemia 9. As our celiac disease page explains, "the tTG-IgA test only works while gluten is still in the diet", so do not cut gluten before the test. Our thyroid panel guide covers reading TSH and free T4 together.
Biotin in blood or other samples taken from patients who are ingesting high levels of biotin in dietary supplements can cause clinically significant incorrect lab test results.US Food and Drug Administration, Safety Communication on biotin and lab tests 5
Does biotin help brittle nails?
Biotin helps brittle nails mainly in people who are short of it, and that group is small 67. The popular claims rest on two small studies from the early 1990s that used high doses, about 80 times the adequate intake. As summarized in Shari Lipner's 2018 review in the Journal of the American Academy of Dermatology, nail thickness rose by 25% in 8 people in one, and in the other 22 of 35 people (63%) said their nails improved, with no comparison group 6. Lipner found no good randomized trials and concluded that the evidence does not support biotin for people who are not deficient 6.
| Supplement | What the studies found | How strong the evidence is |
|---|---|---|
| Biotin | Thicker nails in 8 people (Colombo 1990); 63% reported improvement in 35 people (Hochman 1993), as reviewed by Lipner 6 | Weak: small, uncontrolled, no good trials; skews lab tests |
| Silicon (choline-stabilized orthosilicic acid) | Modest drop in brittleness versus placebo over 20 weeks in women with sun-damaged skin (Barel 2005) 10 | Weak: one small trial, nails a secondary measure |
| Collagen peptides | Nail growth up 12% and broken nails down 42% over 24 weeks (Hexsel 2017) 11 | Weak: open-label, no control group, industry-funded product |
What helps brittle nails, matched to the cause
The fix follows the cause, so the test result decides the next step. Low iron needs a clinician to confirm it and to find out why it is low, because blood loss from the gut or heavy periods has to be ruled out before anyone treats it. Do not start iron tablets on a hunch: extra iron can do harm when stores are normal, and black stools, bleeding after menopause or unexplained weight loss alongside low iron need a doctor soon. When a test confirms low iron, our iron supplement guide covers how to take it well, and our iron-rich foods guide lists the foods to lean on. An underactive thyroid is treated by a doctor, and nails usually recover as thyroid levels settle.
Supplements, with the evidence for nails
For the outside causes, which are the majority, nail care does most of the work:
- Wear cotton-lined rubber gloves for dishes and cleaning, so the nail stays dry and the glove does not trap sweat.
- Rub a moisturizer into nails and cuticles after every wash; creams with urea or lanolin hold water in the nail plate.
- Switch to an acetone-free polish remover and use it less often.
- Give nails breaks from gel and acrylics, and never pick or peel them off.
- Keep nails short and file in one direction with a fine file.
If protein is the gap, the recommended allowance for adults is 0.8 g per kilogram of body weight a day 12, and people losing weight fast often fall below it.
CalculatorProtein calculatorWork out your daily protein in grams, from your weight and goal.Work it out How long until brittle nails get stronger?
Fingernails grow about 3.5 mm a month and toenails about 1.6 mm a month in healthy adults 2. At that rate a whole fingernail takes around 6 months to replace, and a toenail 12 to 18 months. The damaged tip you see today was made months ago, so judge any change by the new nail at the base at 3 months, and by the whole nail at 6.
Who gets brittle nails: menopause, pregnancy, age and diet
- Around and after menopause, lower estrogen can leave skin and nails drier; see our menopause and perimenopause pages.
- In pregnancy, after birth and with heavy periods, iron demand or loss rises, so ferritin is the first test.
- Over 60, nails thin and dry with age, and ridges become common without any deficiency.
- On a vegan or vegetarian diet, iron, zinc and B12 are the nutrients to check; see vegan nutrient deficiencies.
- On keto, after weight-loss surgery or with fast weight loss, protein and calorie gaps thin the nails.
- Swimmers, cleaners, nurses and hairdressers have more wet-dry cycles than anyone, so the cause is usually outside.
- Men get brittle nails about half as often as women 1. Low iron in a man, or in a woman after menopause, is unusual and needs a doctor to look for a cause such as bleeding from the gut.
When brittle nails come with other symptoms
Brittle nails often travel with other signs of the same cause. If yours come with hair loss, fatigue, always feeling cold or tingling hands and feet, those pages walk through the shared tests, and our overview of vitamin deficiency symptoms shows which nutrients cause which signs. Not sure which panel to ask for? See what blood tests should I get.
Citations
- van de Kerkhof PCM, Pasch MC, Scher RK, et al. Brittle nail syndrome: a pathogenesis-based approach with a proposed grading system. J Am Acad Dermatol 2005;53:644-51.
- Yaemsiri S, Hou N, Slining MM, He K. Growth rate of human fingernails and toenails in healthy American young adults. J Eur Acad Dermatol Venereol 2010;24:420-3.
- Cashman MW, Sloan SB. Nutrition and nail disease. Clin Dermatol 2010;28:420-5.
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations. Geneva: WHO, 2020.
- US Food and Drug Administration. UPDATE: The FDA warns that biotin may interfere with lab tests. FDA Safety Communication, 2019 (archived copy).
- Lipner SR. Rethinking biotin therapy for hair, nail, and skin disorders. J Am Acad Dermatol 2018;78:1236-8.
- NIH Office of Dietary Supplements. Biotin: Fact Sheet for Health Professionals.
- Powers JM, Lim MY, Achebe MO, et al. American Society of Hematology 2026 guidelines for diagnosis of iron deficiency. Blood Adv. 2026;10(21):7353-7374.
- National Institute for Health and Care Excellence. Coeliac disease: recognition, assessment and management (NG20). London: NICE, 2015.
- Barel A, Calomme M, Timchenko A, et al. Effect of oral intake of choline-stabilized orthosilicic acid on skin, nails and hair in women with photodamaged skin. Arch Dermatol Res 2005;297:147-53.
- Hexsel D, Zague V, Schunck M, Siega C, Camozzato FO, Oesser S. Oral supplementation with specific bioactive collagen peptides improves nail growth and reduces symptoms of brittle nails. J Cosmet Dermatol 2017;16:520-6.
- Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: National Academies Press, 2005.
- Safer JD. Thyroid hormone action on skin. Dermatoendocrinol 2011;3:211-5.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
