Hair loss can be a sign of thyroid problems, iron deficiency, hormonal imbalances, autoimmune conditions, or the aftereffects of physical or emotional stress. Losing 50 to 150 hairs per day is normal, but noticeable thinning, bald patches, or clumps coming out in the shower point to something worth investigating. The cause is rarely just one thing, and identifying it often starts with looking at what else is happening in your body.
Thyroid Dysfunction
Both an underactive and overactive thyroid can trigger hair loss, but they do it in different ways. When your thyroid is underactive (hypothyroidism), it slows the division of cells responsible for growing hair. Follicles get stuck in their resting phase and don’t cycle back into active growth the way they should. The result is diffuse thinning across the scalp rather than patchy bald spots.
An overactive thyroid (hyperthyroidism) causes a different kind of damage. Excess thyroid hormone ramps up the production of unstable molecules called free radicals, which damage cell membranes in the hair follicle. This oxidative stress weakens hair at the root. In both cases, hair loss tends to improve once thyroid levels are brought back into a normal range, though regrowth can take several months.
Iron and Vitamin D Deficiency
Low iron is one of the most common and most overlooked causes of hair thinning, especially in women. A study of women with non-scarring hair loss found that over half of those experiencing stress-related shedding had serum ferritin levels below 20 ng/mL, and the numbers were even higher among women with pattern thinning (68% below that threshold). Ferritin measures how much iron your body has in storage, and hair follicles are among the first things to suffer when reserves run low.
Vitamin D plays a separate but related role. Hair follicles have receptors that depend on vitamin D to function properly. In one study of people with recurring patchy hair loss, 42% had vitamin D levels in the true deficiency range (under 20 ng/mL), significantly higher than the 29.5% rate seen in healthy controls. Optimal vitamin D for general health starts around 30 ng/mL, and some researchers believe immune system function requires levels above 40 ng/mL. A simple blood test can check both ferritin and vitamin D.
Stress and Physical Trauma
If you’ve noticed a sudden increase in shedding, think back two to three months. That’s the typical delay between a physical or emotional stressor and visible hair loss, a condition called telogen effluvium. The stressor pushes a large number of follicles into their resting phase all at once. Months later, those hairs fall out together.
Common triggers include surgery, high fever, significant weight loss, childbirth, severe emotional stress, and stopping or starting certain medications. Acute episodes typically resolve on their own within six months as follicles cycle back into growth. If shedding persists beyond that window, it may point to an ongoing issue like chronic stress, a nutritional gap, or an undiagnosed condition keeping your body in a prolonged state of alarm.
Hormonal Imbalances and PCOS
In women of reproductive age, hair thinning can be a visible sign of polycystic ovary syndrome (PCOS), the most common hormonal disorder in this group. PCOS involves elevated levels of androgens (hormones like testosterone that are typically higher in men), and one of the places those excess androgens show up is the scalp. The pattern is distinctive: hair thins along the part line and crown while the frontal hairline stays relatively intact.
Other signs that point toward PCOS include excess facial or body hair (hirsutism), acne, irregular periods, and darkened skin patches in body folds (a sign of insulin resistance). Identifying PCOS matters beyond cosmetics. Women with the condition face higher risks of metabolic syndrome, certain cancers, and infertility. Notably, a formal diagnosis doesn’t always require blood work for androgen levels. If clinical signs of excess androgens are already visible, that can be enough.
Autoimmune Conditions
When the immune system mistakenly attacks hair follicles, the result is alopecia areata: smooth, oval patches of hair loss without redness or flaking. The edges of these patches often contain short, tapered hairs that are thin at the base and wider at the tip. These characteristic “exclamation point hairs” help distinguish the condition from other types of hair loss.
Alopecia areata is localized and does not scar the scalp. Systemic autoimmune diseases like lupus can also cause hair loss, but the pattern looks different. Lupus-related hair loss often involves redness or a violet tone at the scalp, irregular pigmentation, and in some forms, permanent scarring that prevents regrowth. If patchy hair loss is accompanied by joint pain, mouth sores, fatigue, or sun sensitivity, those are signals that the immune system may be attacking more than just hair follicles.
Scalp Conditions
Sometimes hair loss starts at the surface. Seborrheic dermatitis, a chronic fungal condition of the scalp, is a common culprit. It triggers excess oil production, which feeds an overgrowth of a yeast called Malassezia that naturally lives on skin. The resulting inflammation damages follicles directly, and the intense itching it causes leads to scratching, which compounds the damage.
The hallmarks are greasy or waxy flakes, redness, and persistent itchiness. Unlike dandruff alone, seborrheic dermatitis actively hinders hair growth by obstructing the follicle. The good news is that hair loss from this condition is generally reversible once the inflammation and yeast overgrowth are brought under control.
How Normal Shedding Differs From a Problem
The 50 to 150 hairs you lose daily are part of a healthy cycle. Each follicle independently grows, rests, sheds, and starts over. You’ll find these hairs on your pillow, in the drain, or on your clothes, and that’s fine. What changes the picture is the volume and pattern.
A simple at-home check: run your fingers through a small section of hair and gently tug. If one or two hairs come away, that’s normal. If several come out with each pass, it’s worth bringing up with a doctor. In a clinical setting, a more standardized version of this test considers more than two hairs pulled from one area to be abnormal. Doctors will also look at whether the loss is diffuse (spread evenly) or patchy, whether the scalp itself looks healthy or inflamed, and whether the hair is breaking mid-shaft or falling out at the root. Each of these details narrows down the cause.
What Gets Tested
When hair loss prompts a medical visit, the workup is usually straightforward. Blood tests commonly check thyroid function, ferritin, vitamin D, and a complete blood count. For women with signs of hormonal excess, tests may include testosterone levels and sex hormone binding globulin, though these aren’t always necessary if clinical signs like hirsutism or acne are already present.
Your doctor will also ask about timing. Hair loss that started two to three months after an illness, surgery, crash diet, or major life event points strongly toward stress-related shedding. Gradual thinning over years suggests a hormonal or genetic pattern. Sudden smooth patches suggest an autoimmune process. The combination of where you’re losing hair, how fast it’s happening, and what else is going on in your body usually leads to a clear answer.