Noticing more hair in your brush or the shower drain lately? You're not imagining it, and you're far from alone. Hair loss is one of the most common concerns I hear about, and it can feel frustrating because it's rarely explained well. The good news: there's real, evidence-based science behind why hair thins and sheds, and a lot of it comes down to factors you can actually influence. Let's walk through what's happening at the follicle level and what genuinely helps.
UNDERSTANDING THE HAIR GROWTH CYCLE
Every hair on your head is cycling through three phases independently of its neighbors: anagen (active growth, which can last years), catagen (a short transition phase), and telogen (a resting phase before the hair sheds and a new one begins). At any given time, most of your hair is in anagen, which is why healthy shedding of 50 to 100 hairs a day is completely normal and not a sign anything is wrong. Hair loss becomes noticeable when something pushes an unusually large number of follicles into telogen at once, or when the anagen phase itself shortens over time, so each hair grows for less time before it's shed.
WHAT'S ACTUALLY BEHIND HAIR LOSS?
Hair loss is almost never caused by just one thing. It's typically a mix of nutrition status, hormones, stress, oxidative and environmental exposure, immune function, genetics, and sometimes medications, all interacting with your individual biology. That's exactly why a one-size-fits-all approach so often falls short, and why identifying which of these factors apply to you is worth the effort. We'll go through each in turn.
NUTRIENTS THAT SUPPORT HAIR HEALTH
Protein
Your hair shaft is roughly 90% keratin, a structural protein built from the sulfur-containing amino acid cysteine, which your body makes from methionine. Deficiencies in essential amino acids like histidine, leucine, and valine show up disproportionately often in people with hair loss. Aim for adequate protein spread across the day (roughly 1.2 to 1.6 grams per kilogram of body weight for most adults) from sources like eggs, fish, poultry, legumes, and other plant proteins.
Essential fatty acids
Deficiencies in omega-3 and omega-6 fatty acids have historically been linked to hair loss, and research suggests they may increase the proportion of follicles sitting in that resting telogen phase rather than actively growing. Cold-water fish, walnuts, chia and flax seeds, and algae-based supplements are good sources.
Vitamin D
Vitamin D plays a role in the signaling pathways that govern hair follicle growth and differentiation, and low serum levels are consistently associated with non-scarring hair loss conditions like telogen effluvium, androgenetic alopecia, and alopecia areata. If you haven't had your level checked recently, it's a reasonable one to ask about.
Iron
Iron is essential for follicle function, and iron depletion is a well-documented, correctable cause of diffuse hair shedding, even before you'd be considered anemic. A serum ferritin below roughly 70 μg/L is often meaningful when hair loss is otherwise unexplained, which is a higher threshold than the lab's standard “normal” cutoff, so this is worth discussing specifically with your provider.
Zinc
Zinc deficiency can cause telogen effluvium, breakage, and thinning, and zinc is one of the most commonly limited nutrients in vegan diets. One clinical tidbit: white spots on the fingernails can sometimes be a visible clue to low zinc status. Food sources include oysters, beef, pumpkin seeds, and legumes.
B vitamins
B12 and folate support DNA synthesis in rapidly dividing hair follicle cells, and B12 repletion has shown benefit specifically in people who are deficient. Biotin is the more famous of the bunch, but the evidence is more limited than the marketing suggests: biotin appears to support hair strength rather than growth, and there's no good evidence it helps unless you're actually deficient, which is uncommon. One important caution: biotin supplementation can interfere with certain lab tests, including troponin (a cardiac marker) and thyroid hormone assays, so let your lab and provider know if you're taking it before bloodwork.
Selenium and antioxidants
Selenium and antioxidant vitamins A, C, and E may help counter the oxidative stress involved in hair loss, and combination antioxidant supplements have shown increased hair density and thickness in some trials. That said, more is not better here: excessive vitamin A, in particular, can paradoxically cause hair loss, so this is a category where working with a practitioner on dosing matters. Green tea and its polyphenol EGCG have also shown a positive effect on hair growth in studies, which is a nice reason to keep that habit if you enjoy it.
HORMONES AND HAIR
As estrogen declines around menopause, so does sex hormone-binding globulin (SHBG), which leaves more free, bioavailable androgens circulating in the blood. That shift can contribute to both scalp thinning and unwanted facial hair growth. Estrogen itself, when it's more abundant, tends to extend the active growth phase of the hair cycle and support the blood supply to follicles, so its decline works against hair health from a couple of different angles at once.
The central driver of the most common pattern of hair loss, androgenetic alopecia, is actually dihydrotestosterone (DHT), not testosterone itself. An enzyme called 5-alpha reductase converts testosterone into DHT, and DHT binds preferentially to follicles in the frontal, temporal, and crown regions of the scalp, which is why thinning tends to follow that pattern while the back and sides of the scalp stay largely resistant (it's also why hair transplants harvest from the back of the head).
Thyroid imbalance, in either direction, can also drive hair changes. Hypothyroidism slows cell division in the hair follicle, reducing the proportion of hairs actively growing, while hyperthyroidism contributes through oxidative stress instead. If your hair loss is diffuse rather than patterned, a full thyroid panel is a reasonable ask.
STRESS, OXIDATIVE STRESS, & ENVIRONMENTAL EXPOSURE
Chronic stress raises cortisol, and cortisol has a direct, unfavorable effect on the follicle: it breaks down hyaluronan and proteoglycans, the substances that help hold hair follicle structures together. Stress hormones can also trigger follicles to shift into that transitional catagen phase earlier than they should, which is part of why a major stressful period often shows up as noticeable shedding two to three months later.
Separately, free radicals generated by UV radiation, air pollution, and smoking directly damage hair proteins, lipids, and even DNA, both inside the follicle and in the visible hair shaft. This oxidative burden increases with age as your body's natural antioxidant defenses decline, which is part of why hair quality tends to change over time even without an obvious trigger. Sun protection, minimizing smoke exposure, and a diet rich in antioxidant-dense produce all help counter this.
GENETICS, IMMUNE FUNCTION, & SPECIAL SITUATIONS
Androgenetic alopecia, the pattern thinning driven by the DHT pathway above, is strongly hereditary and can be inherited from either side of the family. Knowing your family history can help set realistic expectations and point toward earlier, more proactive management if that's a priority for you.
Alopecia areata is a different mechanism entirely: it's autoimmune, involving immune cells that mistakenly attack the hair follicle and force it to abruptly stop growing, which is why it tends to cause sudden, patchy hair loss rather than gradual thinning. It shares genetic risk with other autoimmune conditions, including thyroid autoimmunity, type 1 diabetes, and celiac disease, and if you have one autoimmune condition, it's worth mentioning any hair changes to your provider. On the treatment side, this is an area where the science has moved quickly: a class of medications called JAK inhibitors, which block the specific signaling pathway involved, are now FDA-approved for alopecia areata.
Postpartum shedding is extremely common and, importantly, temporary: the high estrogen of pregnancy keeps an unusually large share of hair in the growth phase, and after delivery, hormone levels reset, sending a wave of follicles into the resting phase at once. This typically peaks around three to four months postpartum and resolves on its own within six to twelve months, though the nutrient support above can help the process along.
Traction alopecia is a mechanical cause worth knowing about: hairstyles that repeatedly pull on the same follicles (tight ponytails, braids, weaves, or extensions) can cause gradual, localized hair loss, most often along the hairline. It's largely preventable by varying your style and easing tension at the root, and it's reversible if caught before the follicle is damaged long-term.
THE SCALP MICROBIOME
Your scalp hosts a genuinely complex ecosystem of bacteria, fungi, and viruses that's shaped by air quality, humidity, pH, sebum production, and your immune response. Seborrheic dermatitis (the cause of stubborn, flaky dandruff) behaves more like a low-grade inflammatory condition than a simple infection. There's also emerging evidence of a distinct scalp bacterial shift in people with alopecia areata, suggesting the microbiome may play a more active role in autoimmune hair loss than previously thought, though that connection is still being studied.
MEDICATIONS THAT CAN AFFECT HAIR
A surprising number of common medications list hair thinning as a possible side effect. This isn't a reason to stop anything on your own, but if you've started a new medication and noticed shedding a few months later, it's worth mentioning to your prescriber rather than assuming it's unrelated.
CategoryExamplesBlood pressure medicationsBeta blockers (especially propranolol), ACE inhibitorsMood and psychiatric medicationsSSRIs, tricyclic antidepressants, lithium, valproic acidHormonal medicationsHormonal contraceptives (especially when switching formulations), anabolic steroidsCancer treatmentChemotherapyOther common categoriesAnticoagulants, retinoids, statins (less consistently), chronic high-dose NSAID use, certain antifungals (e.g., fluconazole), GLP-1 agonists (likely related to rapid weight loss rather than a direct drug effect)
TREATMENT OPTIONS WORTH DISCUSSING WITH YOUR PROVIDER
If nutrition and lifestyle strategies aren't enough on their own, there are well-studied medical options depending on the underlying cause. None of these are a reason to skip the nutrition foundation above, but they're worth knowing they exist.
TreatmentWhat it's generally used forMinoxidilStimulates growth factors and blood flow to the follicle; originally a blood pressure medicationFinasteride / dutasterideBlock the 5-alpha reductase enzyme that converts testosterone to DHT; used for androgenetic alopecia, primarily in menSpironolactoneBlocks the interaction between testosterone and DHT; used for female pattern hair lossTopical cetirizineAn antihistamine with an anti-prostaglandin effect shown to improve hair densityKetoconazole shampooAnti-inflammatory and suppresses DHT activity at the scalpLow-level (red) light therapyA non-drug option with a promising, still-growing evidence base; considered very safeJAK inhibitorsFDA-approved specifically for alopecia areata; block the immune signaling driving that condition
LABS WORTH ASKING ABOUT
If you're working through unexplained hair loss, this is a reasonable starting panel to request:
Ferritin and a full iron panel, ideally targeting a ferritin above 70 μg/L, not just within the standard reference range
25-OH vitamin D, targeting >40 ng/mL
Methylmalonic acid (MMA), a more sensitive marker of B12 status
An omega-3 index or fatty acid panel
TSH, free T4, free T3, and TPO antibodies, to check for both thyroid function and thyroid autoimmunity
ANA, if an autoimmune cause is suspected
Diurnal cortisol testing, if chronic stress is a likely contributor
For those who want to dig deeper, organic acid testing can also assess mitochondrial function and oxidative stress markers, since healthy mitochondrial function turns out to play a meaningful role in normal hair growth too.
THE BOTTOM LINE
Hair loss is common, it's rarely caused by just one thing, and it's genuinely worth investigating rather than just covering up. Start with the foundational nutrients (protein, essential fatty acids, vitamin D, iron, zinc, and B12), address stress and environmental exposure where you can, and get the right labs checked so you're not guessing. If a pattern, autoimmune process, or hormonal driver is involved, there are well-studied treatment options to discuss with your provider. You don't have to figure this out on your own.
Hope this gave you a clearer picture of what's driving your hair health, and a few concrete places to start. If you'd like to build a plan based on your own labs and biochemistry rather than guesswork, reach out and let's talk.
In Health,
Natalie