Telogen effluvium is triggered when a physical or emotional stressor pushes a large number of hair follicles into their resting phase at the same time. Normally, you lose about 50 to 100 hairs a day. With telogen effluvium, that number climbs well above 100, and the shedding typically starts two to three months after whatever disrupted the hair cycle. The causes range from illness and hormonal shifts to nutritional gaps and medication side effects.
How the Hair Cycle Gets Disrupted
At any given time, about 85 to 90 percent of your hair follicles are in their active growth phase, called anagen, while the rest are in the resting (telogen) phase before naturally falling out. When your body encounters a significant stressor, it diverts energy away from non-essential functions like hair growth. Follicles that were actively growing get pushed prematurely into the resting phase. A few months later, those follicles release their hairs all at once, creating the dramatic shedding people notice in the shower or on their pillow.
This delay between the trigger and the shedding is what makes telogen effluvium confusing. By the time your hair starts falling out, the original cause may have resolved weeks ago, making it hard to connect the dots.
Fever and Acute Illness
High fevers are one of the most common physical triggers. Hair shedding after a febrile illness typically begins two weeks to three months after the fever subsides, averaging about nine weeks. The shedding peaks around two to two and a half months and usually continues for roughly six weeks before tapering off. COVID-19 brought widespread attention to this pattern, but any illness with a sustained fever, including flu, pneumonia, and severe infections, can produce the same result.
Surgery, significant blood loss, and physical trauma also fall into this category. The body treats any major physiological shock as a reason to conserve resources, and hair growth is one of the first things it deprioritizes.
Hormonal Shifts After Pregnancy
Postpartum hair loss is one of the most recognizable forms of telogen effluvium. During pregnancy, elevated progesterone keeps hair follicles in a prolonged growth phase, which is why many pregnant women notice thicker, fuller hair. Progesterone increases hair shaft diameter and suppresses signals that would normally transition follicles out of active growth.
After delivery, progesterone drops sharply while prolactin (the hormone that supports breastfeeding) rises. This hormonal reversal pushes a large wave of follicles into the resting phase at the same time. The synchronized shedding that follows, usually starting around three months postpartum, can be alarming but is almost always temporary. Most women see their hair return to its normal cycle within six to twelve months.
Thyroid Imbalances
Both an underactive and overactive thyroid can cause telogen effluvium. Thyroid hormones directly regulate metabolism at the cellular level, and hair follicles have dedicated receptors for the active thyroid hormone T3. This means thyroid dysfunction doesn’t just slow down your metabolism in general; it disrupts hair follicle function directly, independent of broader metabolic effects.
Hair loss from thyroid problems tends to be diffuse, affecting the entire scalp rather than specific patches. It also tends to persist until the underlying thyroid condition is properly managed. If shedding continues for months without an obvious trigger like illness or childbirth, thyroid testing is one of the first steps a doctor will take.
Psychological Stress and Cortisol
Chronic emotional stress triggers telogen effluvium through a different pathway than physical illness, though the end result looks the same. Under sustained stress, the body activates its hormonal stress response, raising cortisol levels. But the damage to hair follicles appears to be more specific than just elevated cortisol.
Research in animal models has shown that chronic stress increases the release of a signaling molecule called Substance P from nerve endings near hair follicles. Substance P activates mast cells in the skin, triggering localized inflammation around the follicle. This inflammatory cascade generates oxidative stress, which disrupts normal hair cycling and pushes follicles into the resting phase prematurely. In studies, blocking this pathway with antioxidants normalized hair growth, reinforcing that it’s the local inflammation, not just general stress hormones, doing the damage.
This helps explain why some people shed hair during prolonged periods of anxiety, grief, work burnout, or caregiving stress, even without any physical illness.
Iron Deficiency and Low Ferritin
Low iron stores are a well-established nutritional cause of telogen effluvium, particularly in women of reproductive age. Ferritin, the protein that stores iron, serves as a reliable marker. In one case-control study, women with telogen effluvium had an average ferritin level of 16.3 ng/mL, compared to 60.3 ng/mL in women without hair loss. Using a threshold of 30 ng/mL or below, researchers found that low ferritin made a person 21 times more likely to have telogen effluvium.
What makes this tricky is that standard lab reference ranges for ferritin often list anything above 10 or 12 ng/mL as “normal.” You can technically have ferritin within the lab’s reference range and still be low enough to trigger shedding. If you’re experiencing unexplained hair loss, asking specifically about your ferritin number (not just whether it’s flagged as abnormal) is worth doing.
Other nutritional deficiencies linked to telogen effluvium include zinc, vitamin D, and protein. Crash diets, restrictive eating patterns, and bariatric surgery are common triggers because they can deplete multiple nutrients at once.
Medications That Trigger Shedding
Several classes of medication can cause telogen effluvium as a side effect. Retinoids, which are vitamin A derivatives used for acne and psoriasis, are among the most commonly reported. Retinoid-related hair loss is dose and duration dependent, meaning higher doses taken over longer periods carry more risk. Even excessive vitamin A supplementation beyond the recommended daily intake (around 3000 IU for men, 2333 IU for women) can contribute.
Other drug classes linked to telogen effluvium include:
- Anticoagulants (blood thinners)
- Beta-blockers (used for high blood pressure and heart conditions)
- Antidepressants, particularly SSRIs
- Hormonal contraceptives, especially when starting or stopping them
Medication-induced shedding follows the same delayed timeline as other triggers, appearing weeks to months after starting the drug. This lag makes it easy to overlook the connection, especially if you started a new medication and assumed the adjustment period was over.
How It Gets Diagnosed
There’s no single lab test for telogen effluvium. Diagnosis is largely based on your history and a physical exam. One of the simplest clinical tools is the hair pull test: a doctor gently tugs on a small clump of 40 to 60 hairs. In a healthy scalp, this typically pulls out two to three hairs. With active telogen effluvium, four to six or more hairs come out, and they’ll have small white bulbs at the root end, confirming they were in the resting phase.
Blood work is usually ordered to rule out underlying causes: thyroid function, ferritin, vitamin D, zinc, and a complete blood count. The goal isn’t just to confirm telogen effluvium but to identify whether a treatable condition is driving it.
Acute vs. Chronic Telogen Effluvium
Most cases of telogen effluvium are acute, meaning the shedding lasts a few months and resolves on its own once the triggering event passes. Hair typically starts regrowing within six months, though it can take up to a year for density to fully recover since new hairs grow at roughly half an inch per month.
When shedding persists beyond six months without a clear ongoing trigger, it’s classified as chronic telogen effluvium. This form is more common in middle-aged women and can fluctuate in intensity over years. The exact cause is often harder to pin down. In some cases, multiple low-level stressors (borderline nutritional deficiencies, mild thyroid dysfunction, chronic stress) overlap, each insufficient on its own to cause hair loss but collectively enough to keep follicles cycling abnormally. Addressing these contributing factors one by one is usually the most effective approach.