Brittle hair that breaks easily and falls out more than usual typically points to one of a few causes: a nutritional deficiency, physical or chemical damage to the hair shaft, hormonal changes, or a stress response your body mounted weeks or months ago. Sometimes it’s a combination. The good news is that most causes are reversible once you identify what’s going on.
A normal head sheds between 50 and 150 hairs per day. If you’re consistently seeing more than that, noticing thinning patches, or finding short broken strands on your pillow and clothes, something is disrupting either the strength of your hair shaft or the growth cycle at the root.
Breakage vs. Shedding: How to Tell the Difference
Before anything else, it helps to figure out whether your hair is breaking mid-shaft or falling out from the root. The distinction matters because the causes and fixes are different. Pull a few loose strands from your brush and place them on a white surface. If most strands are full-length with a small, smooth white bulb at one end, that’s shedding from the follicle. The bulb (sometimes called a “club root”) means the hair completed its growth cycle or was pushed out early. If the strands are shorter than your hair length, have no bulb, and the ends look jagged, frayed, or split, that’s breakage. The shaft snapped somewhere along its length because it was too weak to hold together.
Many people experience both at the same time, which is why the hair can feel like it’s thinning from every direction. Brittleness leads to breakage. Root-level problems lead to shedding. Nutritional deficiencies can cause both.
Low Iron Is One of the Most Common Culprits
Iron deficiency is one of the most frequently overlooked reasons for hair that thins and breaks. Your follicles need iron to fuel the rapid cell division that builds each strand. When your body’s iron stores drop, it prioritizes vital organs and diverts resources away from hair.
The tricky part is that standard blood tests can miss the problem. Most labs flag anemia only when hemoglobin drops below 12.0 g/dL, but research published in the Tzu Chi Medical Journal found that hair needs more iron than that threshold suggests. Ferritin, the protein that stores iron, needs to be at least 40 to 60 ng/mL for healthy hair growth. That corresponds to a hemoglobin level of about 13.1 to 13.8 g/dL. In other words, your blood count can look “normal” while your ferritin is still too low to support your hair. If you’re getting tested, ask specifically for a ferritin level and look for a result of at least 60 ng/mL.
Women who menstruate, vegetarians, frequent blood donors, and endurance athletes are especially prone to iron stores that hover in this gray zone.
Other Nutritional Gaps That Weaken Hair
Iron isn’t the only nutrient your hair depends on. Several others play direct roles in building and maintaining strong strands.
Protein. Hair is almost entirely made of keratin, a structural protein. Your body needs a steady supply of amino acids to keep producing it. The baseline recommendation is about 0.8 grams of protein per kilogram of body weight per day. For a 150-pound person, that’s roughly 55 grams daily. If you’re consistently falling short, through crash dieting, restrictive eating, or simply skipping meals, your body slows keratin production and existing hair becomes dry and fragile.
Vitamin D. Vitamin D receptors sit directly on the cells that drive the hair growth cycle. The active form of vitamin D helps push resting follicles into their active growth phase and keeps them there longer. It also supports the cells at the base of the follicle that anchor and nourish each strand. Low vitamin D is extremely common, particularly in people who spend most of their time indoors or live at higher latitudes.
Zinc. Zinc supports the enzymes involved in building hair protein and keeping oil glands around the follicle functioning properly. Low zinc levels are associated with thinning hair, and in clinical studies, supplementation over 12 weeks improved hair regrowth in people whose levels were low. A simple blood test can check your zinc status.
Biotin. True biotin deficiency is rare in people eating a varied diet, but it does cause brittle hair and nails when it occurs. It’s more likely if you’re taking certain medications, consuming raw egg whites regularly, or have a gut absorption issue.
Stress Can Trigger Delayed Shedding
If your hair started falling out seemingly out of nowhere, think back two to three months. A major stressor, whether physical or emotional, can push a large percentage of your hair follicles into their resting phase all at once. This condition, called telogen effluvium, causes noticeable shedding that typically begins one to six months after the triggering event, with three months being the most common delay.
Common triggers include surgery, high fevers, significant weight loss, childbirth, severe emotional stress, stopping birth control, and major illness. The shedding itself usually lasts less than six months. After that, new growth begins, though it can take six months to a year before your hair feels full again. The reassuring part is that follicles aren’t damaged. They’re just temporarily paused. Recovery happens on its own once the trigger resolves.
Heat and Chemical Damage to the Hair Shaft
If your hair is brittle but not necessarily falling out from the root, the problem may be external damage. Hair’s strength comes from disulfide bonds, which are chemical bridges between sulfur-containing amino acids that hold the protein structure together. Both heat and chemical treatments can break these bonds.
Wet hair begins to sustain structural damage at temperatures between 120°C and 150°C (roughly 250°F to 300°F). Dry hair is more resilient and can tolerate up to about 240°C (464°F) before the protein denatures. Most flat irons and curling irons operate well within the danger zone for wet or damp hair, and many reach temperatures that damage even dry hair with repeated exposure. If you use heat tools daily, cumulative damage weakens the shaft until strands snap.
Chemical relaxers and perms work by deliberately breaking those same disulfide bonds to reshape the hair. Research analyzing the amino acid content of chemically relaxed hair found significant reductions in cystine, the amino acid responsible for those strength-giving bonds. The result is a permanently weakened shaft that’s more fragile and prone to snapping. Each subsequent chemical treatment compounds the damage.
Color treatments, bleach, and even harsh sulfate shampoos strip the outer cuticle layer that protects the inner structure of the strand. Once the cuticle is compromised, moisture escapes, the hair dries out, and brittleness follows.
Thyroid and Hormonal Causes
Both an underactive and overactive thyroid gland can cause hair that becomes dry, coarse, and thin. Thyroid hormones directly regulate the hair growth cycle, and when levels are off, follicles can stall in their resting phase. Hair may also become unusually brittle and break more easily. If you’re also experiencing fatigue, unexplained weight changes, cold sensitivity, or skin changes alongside your hair issues, a thyroid panel is worth requesting.
Hormonal shifts related to menopause, polycystic ovary syndrome (PCOS), or changes in birth control can also alter hair texture and density. Androgens, a class of hormones that includes testosterone, can miniaturize hair follicles over time, producing finer, weaker strands that eventually stop growing altogether. This pattern tends to show as gradual thinning at the part line or temples rather than sudden shedding.
What to Do About It
Start by looking at the pattern. If you’re finding short, broken pieces without a root bulb, focus on reducing heat and chemical exposure. Lower your styling tool temperatures to below 300°F, never apply heat to damp hair, and use a heat protectant. Give chemically treated hair a break between sessions, and switch to gentler, sulfate-free cleansers.
If full-length strands with white bulbs are coming out in greater numbers, the issue is likely internal. A blood panel checking ferritin, vitamin D, zinc, thyroid hormones, and a complete blood count gives the clearest picture. Remember to look at ferritin specifically, not just hemoglobin, and aim for a level of 60 ng/mL or above.
Evaluate your diet honestly. Getting enough protein, iron-rich foods, and a variety of vitamins through meals is more effective and safer than mega-dosing supplements without knowing your levels. Supplementing iron or zinc when you’re not actually deficient can cause its own problems.
If the shedding started a few months after a major life event, illness, or dramatic diet change, you’re likely dealing with a stress-related shed that will resolve on its own. Resist the urge to aggressively treat it with products. Focus on adequate nutrition, gentle handling, and patience. Most people see meaningful regrowth within six to twelve months once the underlying cause is addressed.