Why Is My Hair Getting Thinner?

Noticing more hair in your brush, or seeing your scalp more than you used to? Hair thinning is one of the most common concerns people bring to their doctors — and one of the most frustrating, because there are many possible causes and they require very different approaches.

Here's how to think through what might be driving yours.

How Hair Growth Works (and What Can Disrupt It)

Hair grows in cycles. Each follicle goes through a growth phase (anagen, lasting 2-7 years), a transition phase (catagen, about 2 weeks), and a resting phase (telogen, about 3 months) before the hair sheds and a new cycle begins.

At any point, roughly 90% of your scalp follicles are in the growth phase. Anything that disrupts this cycle — stress, nutritional deficiencies, hormonal shifts, illness — can push more follicles into the resting and shedding phase simultaneously, producing noticeable thinning.

The Most Common Causes of Hair Thinning

Androgenetic alopecia (pattern hair loss)

This is the most common cause of hair loss in both men and women. In men, it typically produces a receding hairline and thinning at the crown. In women, it usually manifests as diffuse thinning across the top of the scalp while the hairline largely remains intact.

The underlying cause is a hormone called dihydrotestosterone (DHT), a byproduct of testosterone. DHT causes susceptible follicles to shrink progressively, producing finer and shorter hairs until they stop producing visible hair altogether.

There's a strong genetic component — if your close relatives lost their hair, your risk is meaningfully higher.

Telogen effluvium

This is a temporary form of hair shedding triggered by a physical or emotional stressor. The hair loss typically occurs 2-4 months after the triggering event, which makes the connection easy to miss.

Common triggers include:

• Major illness or surgery
• Significant emotional stress
• Childbirth (postpartum hair loss is a form of this)
• Crash dieting or significant weight loss
• Starting or stopping certain medications

The good news: telogen effluvium usually resolves on its own within 6-9 months once the trigger is addressed.

Thyroid disorders

Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can cause diffuse hair thinning. The hair loss is often accompanied by other symptoms — fatigue and weight changes with hypothyroidism, anxiety and heart palpitations with hyperthyroidism. Thyroid function is one of the first things to check when hair thinning is unexplained.

Iron deficiency

Iron is essential for hair follicle function. Iron deficiency — even without full anemia — is a common and frequently overlooked cause of hair thinning, particularly in women with heavy periods or those following plant-based diets.

Nutritional deficiencies

Beyond iron, deficiencies in zinc, vitamin D, biotin, and protein can all contribute to hair thinning. Severe calorie restriction depletes the nutrients available to non-essential processes like hair growth.

Hormonal changes

Shifts in estrogen and progesterone levels — during pregnancy, after delivery, with certain birth control changes, or during perimenopause — can all affect hair growth cycles significantly.

Alopecia areata

An autoimmune condition where the immune system attacks hair follicles, producing patches of hair loss rather than diffuse thinning. This requires a different treatment approach than androgenetic alopecia.

Medication side effects

Many medications list hair loss as a side effect, including certain blood pressure drugs, antidepressants, anticoagulants, and acne medications.

How to Figure Out What's Behind Yours

The single most useful first step is blood work. A physician can check thyroid function, iron studies, ferritin, vitamin D, zinc, and hormonal markers — ruling out or confirming several common causes with a single panel.

Your pattern of loss matters too. Is it overall thinning, or concentrated somewhere specific? Is it shedding (lots of loose hairs) or actual follicle loss (widening part line, visible scalp)?

A physician who takes a careful history — medications, diet, recent life events, family history — combined with basic labs can usually identify the most likely cause and direct appropriate treatment.

Ready to talk to a real doctor? Get started with Coral Health today.