Baldness happens when hair follicles either shrink, shut down, or get destroyed, and the cause is almost always one of a handful of well-understood conditions. The most common by far is genetic sensitivity to hormones, which accounts for the vast majority of hair loss in both men and women. But nutritional gaps, autoimmune reactions, stress, thyroid problems, and scarring conditions can also be responsible. Figuring out which one applies to you determines whether your hair can grow back and what to do about it.
Genetic Hair Loss: The Most Common Cause
If your hair has been gradually thinning over months or years, especially in a predictable pattern (receding hairline, thinning crown, or widening part), you’re likely dealing with androgenetic alopecia. This is pattern baldness, and it affects roughly half of all men by age 50 and a significant percentage of women as well.
The mechanism is straightforward. Your body converts testosterone into a more potent hormone called DHT. In people with a genetic predisposition, DHT binds to receptors in certain scalp follicles and gradually shrinks them. The hairs produced become finer, shorter, and lighter with each growth cycle until eventually the follicle produces nothing visible at all. This process is called follicular miniaturization.
The key detail: your DHT levels don’t need to be abnormally high. People who go bald typically have normal hormone levels. The difference is that their follicles are inherited to be more sensitive to those hormones. This sensitivity is polygenic, meaning it comes from a combination of gene variants, many of which sit on or near the androgen receptor gene. You can inherit the tendency from either parent, not just your mother’s side as the old myth suggests.
A 12-month evaluation of 502 men using a combination of FDA-approved medications found that 92.4% were stable or improved, with 57.4% showing visible regrowth. These results reflect currently available treatments that work by either blocking DHT production or stimulating follicle activity. The earlier you start, the more follicles are still alive to respond.
Stress and Illness Can Trigger Sudden Shedding
If your hair loss came on suddenly and you’re losing clumps in the shower or finding hair all over your pillow, the likely culprit is telogen effluvium. This is a temporary condition where a large number of follicles get shocked into their resting phase all at once, then shed two to three months later. It’s why you might notice dramatic thinning months after the stressful event, not during it.
The list of triggers is long: high fevers, major surgery, severe emotional stress, crash dieting, significant blood loss, childbirth, thyroid disorders, and certain medications including some blood pressure drugs, anticonvulsants, and hormonal contraceptives. COVID and other serious infections are well-documented triggers. Even rapid weight loss from calorie restriction can do it.
The good news is that telogen effluvium reverses on its own once the trigger is removed. Hair shedding typically stops within three to six months, and new growth becomes visible three to six months after that. Full cosmetic recovery can take 12 to 18 months. The follicles aren’t damaged, just temporarily disrupted.
Nutritional Deficiencies That Cause Hair Loss
Your hair follicles are metabolically demanding, and they’re among the first things your body deprioritizes when nutrients run low. Three deficiencies show up most consistently in hair loss research.
Iron: Low iron storage is one of the most common correctable causes of hair thinning, particularly in women. Doctors measure this with a ferritin blood test. While clinical iron deficiency anemia starts at very low levels, hair loss can begin when ferritin drops below 40 ng/dL. Some specialists recommend keeping levels above 70 ng/dL to reverse severe shedding.
Zinc: Studies show a strong correlation between zinc levels below 70 µg/dL and hair loss. Zinc deficiency is more common in people with restricted diets, digestive conditions that impair absorption, or chronic illness.
Vitamin D: In one study, nearly 97% of hair loss patients were vitamin D deficient (below 20 ng/mL), compared to 73% of healthy controls. Vitamin D plays a role in the hair growth cycle, and low levels are increasingly recognized as a contributing factor.
If your hair loss is diffuse rather than patterned, and especially if you have fatigue or other symptoms, a simple blood panel can check all three of these along with thyroid function.
Autoimmune Hair Loss: Patchy and Unpredictable
Alopecia areata looks different from pattern baldness. It typically produces smooth, round bald patches that appear quickly, sometimes within days. The underlying cause is your immune system mistakenly attacking your own hair follicles. Specifically, certain immune cells lose the ability to recognize follicle cells as “self” and launch an inflammatory assault on the hair bulb during its active growth phase.
The condition has both genetic and environmental components. Identical twins don’t always share it, which confirms that triggers beyond genetics play a role. Viral infections, particularly ones that ramp up interferon production, can set it off or make it worse. Emotional stress is another commonly reported trigger, though the exact connection is still being mapped.
Alopecia areata can range from a single coin-sized patch to total scalp or body hair loss. Unlike pattern baldness, the follicles aren’t destroyed. They’re suppressed. This means regrowth is possible, and many people experience spontaneous recovery. For severe cases, three targeted medications have received FDA approval since 2022. These drugs work by interrupting the specific immune signaling pathway that drives the attack on follicles, and clinical trials showed significant regrowth in patients with severe disease.
Scarring Alopecia: When Hair Loss Is Permanent
This is the type of baldness where timing matters most. In scarring (cicatricial) alopecia, inflammation destroys the stem cells that generate new hair, and the follicle gets replaced by scar tissue. Once that happens, no treatment can bring the follicle back.
Warning signs include scalp redness, burning, itching, or pain in the area of hair loss. The affected skin may look shiny or smooth, and the follicle openings (pores) may be visibly absent. One hallmark that doctors look for is the loss of the small oil glands attached to each follicle. Conditions like frontal fibrosing alopecia, which causes a slowly receding hairline often accompanied by eyebrow thinning, fall into this category.
Scarring alopecias are less common than other types but important to catch early. Treatment focuses on stopping the inflammatory process before more follicles are lost, rather than regrowing what’s already gone.
How Doctors Figure Out Which Type You Have
A dermatologist can often identify the type of hair loss just by looking at the pattern, the scalp surface, and the quality of remaining hairs. But when the cause isn’t obvious, blood work helps narrow things down.
- Ferritin: checks your iron stores, with levels below 30-40 ng/dL flagging deficiency as a likely contributor
- TSH (thyroid-stimulating hormone): screens for thyroid disease, which can cause diffuse thinning along with fatigue, weight changes, or muscle weakness
- Androgen levels: particularly useful in women with thinning hair and no clear bald patches, where elevated testosterone or related hormones may point to a hormonal imbalance
- Vitamin D and zinc: checked when nutritional causes are suspected
In some cases, a small scalp biopsy helps distinguish between scarring and non-scarring types, which is critical because the treatment approach is completely different. If you’re losing hair rapidly, in patches, or with scalp symptoms like pain or redness, those are reasons to get evaluated sooner rather than later. Gradual thinning gives you more time, but earlier intervention consistently produces better outcomes regardless of the cause.