Eyebrow thinning happens for a range of reasons, from hormonal shifts and nutritional deficiencies to overgrooming and autoimmune conditions. The cause shapes where and how you lose hair, so paying attention to the pattern of thinning can point you toward the right explanation.
Thyroid Problems and the Outer Eyebrow
One of the most recognizable patterns of eyebrow thinning is loss concentrated in the outer third of the brow. This is called the Hertoghe sign, and it’s closely associated with an underactive thyroid (hypothyroidism). Your thyroid gland produces hormones that regulate many body processes, including hair growth. When it underperforms, those hormone levels drop and hair follicles can stall.
The exact reason the outer third thins first isn’t fully understood. One theory involves a buildup of certain compounds between cells that triggers inflammation, disrupting the skin structure around follicles. Another points to dysfunction in the autonomic nervous system, which controls many unconscious body processes. Both hypothyroidism and hyperthyroidism can cause thinning, so the issue isn’t limited to one direction of thyroid dysfunction. If your thinning is noticeably worse at the tail end of your brows, a thyroid check is a logical first step.
Hormonal Changes During Menopause
Hair growth is under hormonal control, and the hormonal landscape shifts significantly around menopause. As estrogen production declines, the balance tips toward androgens (hormones like testosterone that both men and women produce). This shift can trigger a pattern of hair thinning similar to what happens on the scalp with androgenetic alopecia. Eyebrows thin with age generally, as all body hair growth slows, but the hormonal changes of perimenopause and menopause can accelerate that process noticeably.
Frontal Fibrosing Alopecia
Frontal fibrosing alopecia (FFA) is a condition that primarily affects women after menopause, most often after age 50. The main symptom is a receding hairline along the front and sides of the scalp, but eyebrow loss is the other hallmark. It affects between 80% and 90% of women with FFA, and in many cases, eyebrow thinning appears before any noticeable scalp hair loss.
Early signs can also include flat, dark brown or gray patches of skin on the face and neck, along with itching or pain on the scalp. FFA is a scarring condition, meaning the hair follicles it destroys don’t recover on their own. Diagnosis typically requires a physical exam and a skin biopsy. If your eyebrow thinning is paired with a gradually retreating hairline, FFA is worth investigating.
Alopecia Areata
Alopecia areata is an autoimmune condition where your immune system mistakenly attacks hair follicles. It can affect the scalp, eyebrows, eyelashes, or any combination. In the eyebrows, it often shows up as patchy gaps rather than uniform thinning, though in severe cases it can cause total eyebrow loss.
Research in the British Journal of Dermatology found a clear correlation between the severity of scalp involvement and eyebrow loss. People with minimal scalp hair loss tended to have intact eyebrows, while those with the most extensive scalp loss (95% to 100%) more commonly had complete eyebrow loss. That said, alopecia areata is unpredictable. Some people lose eyebrow hair without significant scalp involvement, and the condition can come and go in cycles.
Nutritional Deficiencies
Iron deficiency is one of the more common nutritional causes of hair thinning, including in the eyebrows. Iron helps produce hemoglobin, which carries oxygen to hair follicles. When iron is low, follicles don’t get the support they need to grow normally. Low ferritin (the protein that stores iron) is a subtler version of the same problem: your reserves are depleted even if your iron levels look borderline acceptable on a standard test.
Anemia from iron deficiency can trigger a condition called telogen effluvium, where a larger-than-normal proportion of hair follicles shift into their resting phase at once. The result is diffuse thinning across the scalp and body hair, including brows. Other nutrient shortfalls, particularly zinc, biotin, and vitamin D, can contribute as well, though iron and ferritin are the most commonly tested.
Overgrooming and Follicle Damage
Years of aggressive plucking, waxing, or threading can damage eyebrow follicles to the point where regrowth slows or stops. Eyebrow hair follows a growth cycle with three phases: an active growth phase lasting two to three months, a brief transition phase of two to three weeks, and a resting phase of two to three months. Under normal conditions, plucked hairs cycle back through and regrow.
But repeated trauma to the follicle can weaken it permanently. If you’ve been overgrooming for years and notice that your brows never quite fill back in the way they used to, the follicles may have sustained enough damage to extend recovery time significantly, or in some cases, to stop producing visible hair altogether. Giving your brows a long break from grooming (at least several full growth cycles, so roughly four to six months) is the simplest way to see how much recovery is possible.
What Testing Looks Like
If your eyebrow thinning doesn’t have an obvious cause like overgrooming, a few blood tests can help narrow things down. A thyroid-stimulating hormone (TSH) test evaluates whether your thyroid is over- or underperforming. Iron and ferritin studies check whether your body has enough iron and adequate reserves. A complete blood count (CBC) screens for anemia and other conditions that could affect hair growth by measuring red and white blood cells, hemoglobin, and platelets.
These tests are straightforward and widely available. Together, they cover the most common systemic causes of hair thinning. If results come back normal and the pattern of loss is unusual (patchy, one-sided, or accompanied by scarring or skin changes), a dermatologist can evaluate for autoimmune conditions or scarring alopecias that require a skin biopsy to diagnose.
Treatment Options for Regrowth
For thinning caused by an underlying condition like hypothyroidism or iron deficiency, treating the root cause is the most effective path. Once thyroid levels normalize or iron stores rebuild, hair follicles often resume their normal cycle, though regrowth can take several months to become visible.
For thinning without a clear reversible cause, topical treatments can help. Minoxidil, available over the counter in 2% to 5% concentrations, has shown measurable results in clinical trials. In one study, participants who applied a 2% concentration to their eyebrows for 16 weeks saw better hair regrowth compared to a placebo group. Side effects are generally mild: occasional burning, dryness, itchiness, or redness at the application site.
A prescription option is bimatoprost, a prostaglandin analog originally developed for glaucoma that was found to promote hair growth as a side effect. In clinical trials, a 0.03% concentration improved eyebrow fullness in roughly 78% to 84% of participants by month seven, depending on how frequently it was applied. Improvements in hair darkness appeared as early as month one, with noticeable fullness gains by month two. Bimatoprost requires a prescription and isn’t approved specifically for eyebrows, so it’s used off-label.
Both treatments require consistent, long-term use. Stopping either one typically means the new growth gradually reverts, since neither changes the underlying biology of the follicle permanently.