Outpatiented · Case Knowledge
You are losing more hair than you should. Not gradual thinning over decades. Noticeable shedding, a thinner ponytail, hair on the pillow. The doctor checked thyroid and maybe ferritin. Everything was normal. You were told it might be stress. It might be. Stress is actually one of the causes. But the mechanism is specific, the timeline is specific, and the ferritin threshold that matters for hair is almost never correctly interpreted.
How Hair Loss Works
Hair grows in cycles. Each follicle cycles through an active growth phase (anagen, lasting two to six years), a transition phase (catagen), and a resting phase (telogen), after which the hair sheds and the cycle restarts. At any given time, roughly 85 to 90 percent of scalp hairs are in anagen and 10 to 15 percent are in telogen.
When the body experiences a significant stressor, whether physical illness, surgery, rapid weight loss, childbirth, crash dieting, severe emotional stress, nutritional deficiency, or hormonal shift, a proportion of anagen hairs shift prematurely into telogen. The shedding happens when those hairs complete the telogen phase and release, which takes approximately two to four months after the triggering event.
This delay is why people rarely connect the stressor to the shedding. The illness was three months ago. The crash diet ended in the spring. By the time the hair falls out, the event feels unrelated. But the hair follicle recorded it and responded on its own timeline.
This pattern is called telogen effluvium. It is the most common cause of diffuse hair shedding in adults and is consistently underdiagnosed because the timing obscures the cause.
The stressor that is causing your hair to fall out now
happened three months ago.
The Actual Causes
The standard hair loss workup typically includes TSH and sometimes a CBC and ferritin. If those are normal, the workup is considered complete. The following causes either require a better test, a better interpretation of a test already run, or a careful medication history.
Iron is required for DNA synthesis in rapidly dividing cells. Hair follicles are among the most rapidly dividing cells in the body. Ferritin is the iron storage protein and is a more sensitive marker of iron depletion than hemoglobin or serum iron. Standard lab reference ranges flag ferritin as low below 10 to 12 ng/mL. Research on hair loss specifically and consistently associates diffuse shedding with ferritin below 50 to 70 ng/mL. A ferritin of 22 ng/mL will be reported as normal and is almost certainly contributing to hair shedding. The lab report will say normal. The hair follicle disagrees. Ask for your actual ferritin number. If it is below 50, the finding is worth acting on regardless of what the report says.
Both hypothyroidism and hyperthyroidism cause hair loss. TSH within the normal range does not rule out thyroid-related hair loss. Hashimoto's thyroiditis, the most common autoimmune thyroid condition, can produce hair loss through TPO antibody-driven follicular inflammation during years when TSH is still normal. Subclinical hypothyroidism with TSH between 2.5 and 4.5 is associated with hair thinning in symptomatic women. Free T3 and Free T4 complete the picture TSH cannot. TPO antibodies identify Hashimoto's before TSH becomes abnormal. A TSH-only thyroid screen is not an adequate hair loss workup.
Any significant physiological or psychological stressor can trigger a wave of follicles to shift from growth into resting phase. The shedding follows two to four months later. Common triggers include major illness or surgery, COVID-19 (post-COVID telogen effluvium became one of the most widely reported long COVID symptoms), significant caloric restriction or crash dieting, childbirth (postpartum shedding typically peaks at three to four months postpartum), major psychological stress, and starting or stopping hormonal contraceptives. Telogen effluvium is self-limiting in most cases if the trigger resolves. When the trigger is ongoing (chronic stress, persistent nutritional deficiency), the shedding continues.
Androgenetic alopecia is the most common cause of progressive patterned hair loss. In women it presents as diffuse thinning at the crown and widening part rather than the receding hairline pattern seen in men. It is driven by genetic sensitivity of hair follicles to dihydrotestosterone (DHT), a testosterone metabolite. PCOS is a common associated condition in women with androgenetic loss. DHT pattern loss responds to different interventions than nutritional deficiency or telogen effluvium, including minoxidil, antiandrogens (spironolactone), and DHT-blocking supplements. Distinguishing the pattern matters for knowing what will actually help.
Hair loss is a documented side effect of a significant number of commonly prescribed medications and is almost never discussed at prescribing. Blood thinners (warfarin, heparin) cause hair loss through disruption of the clotting and growth factor systems. Beta blockers produce telogen effluvium in a subset of users. Some antidepressants, particularly those that affect serotonin strongly, are associated with hair loss. Hormonal contraceptives, particularly those with higher androgenic progestins, can worsen androgenetic loss. Statins, isotretinoin, lithium, valproic acid, and certain blood pressure medications are also on the list. If hair loss started or worsened after beginning a medication, that timing is information and warrants a review of every medication on the list.
The scalp has its own microbiome. Malassezia yeast overgrowth on the scalp drives seborrheic dermatitis and dandruff and is associated with follicular inflammation that impairs hair growth. Gut dysbiosis impairs nutrient absorption broadly, which affects every system downstream including hair follicles. Zinc, biotin, and amino acid deficiencies from poor gut absorption can contribute to hair loss without showing dramatically on standard panels. The scalp and gut connection is rarely discussed in a standard hair loss conversation.
Standard lab reference ranges for ferritin were not designed around hair biology. They were designed to detect iron deficiency anemia at the population level. The threshold below which ferritin is flagged as low (approximately 10 to 12 ng/mL in most labs) is where severe depletion begins to affect red blood cell production.
Hair follicles are sensitive to iron status at a much higher ferritin level. Research consistently shows that diffuse hair shedding is associated with ferritin below 50 ng/mL. Some studies suggest the threshold for optimal hair growth is closer to 70 ng/mL.
The gap between 12 ng/mL (lab's normal threshold) and 70 ng/mL (hair-optimal threshold) is large. A ferritin of 18, 25, or 35 ng/mL will be reported as normal. It is also likely contributing to hair loss. The number on the report is not the relevant number. The number relative to what your hair follicles need is.
What to Do
Hair loss investigation starts with pattern (diffuse vs. patterned, vertex vs. overall), timeline (when did it start, what happened two to four months before), and a complete medication review. Then the right tests.
Questions People Actually Ask
Why is my hair falling out with no known cause?
The most common explanation is that the cause exists but was not identified in the workup. Standard hair loss workups typically check TSH and sometimes ferritin. If those are reported as normal, the investigation often stops.
But TSH alone misses subclinical thyroid dysfunction and Hashimoto's. Ferritin reported as normal is frequently below the threshold hair follicles actually need. Telogen effluvium from a stressor three months prior is not tested for at all. Medication side effects are almost never reviewed.
Diffuse hair shedding with a reportedly normal workup most commonly traces to low ferritin (below 50 to 70 ng/mL despite being reported normal), incomplete thyroid evaluation, a telogen effluvium trigger two to four months before shedding started, or medication side effects.
What ferritin level is needed to stop hair loss?
Research on ferritin and hair loss consistently places the threshold for associated shedding below 50 ng/mL, with some studies suggesting optimal hair growth requires ferritin at or above 70 ng/mL.
Standard lab reference ranges flag ferritin as low below approximately 10 to 12 ng/mL. A ferritin of 18, 25, or 35 ng/mL will be reported as normal and is likely contributing to hair shedding. Ask for your actual ferritin number and compare it to 50 and 70 ng/mL, not just the lab's flag.
Can stress cause hair loss months later?
Yes. This is telogen effluvium and the timing is the key feature that makes it confusing. Significant stress, illness, surgery, childbirth, or rapid weight loss causes a wave of hair follicles to shift from active growth into resting phase. The hair then sheds two to four months later when those follicles complete the resting phase.
This delay is why people rarely connect the stressor to the shedding. The illness was three months ago and feels unrelated. The crash diet ended months before the hair started falling. The hair follicle recorded the stressor and responded on its own timeline, not the timeline that makes intuitive sense.
Can my thyroid cause hair loss even if TSH is normal?
Yes, through two mechanisms. First, Hashimoto's thyroiditis can produce hair loss through autoimmune follicular inflammation during years when TSH is still normal and before overt hypothyroidism develops. TPO antibodies identify Hashimoto's and are not included in a TSH-only screen.
Second, subclinical hypothyroidism with TSH in the upper normal range (2.5 to 4.5) is associated with hair thinning in symptomatic women in the functional medicine literature. Free T3, the active thyroid hormone, can be low with a normal TSH. A complete thyroid panel includes TSH, Free T3, Free T4, and TPO antibodies.
Can medication cause hair loss?
Yes, and this is more common than is routinely discussed at prescribing. Blood thinners (warfarin, heparin), beta blockers, some antidepressants, hormonal contraceptives (particularly those with androgenic progestins), statins, isotretinoin, lithium, valproic acid, and some blood pressure medications all have hair loss as a documented side effect.
The mechanism varies. Some drugs cause telogen effluvium directly. Hormonal contraceptives with androgenic activity can worsen androgenetic loss. If hair loss began or worsened within weeks to months of starting a medication, that correlation warrants investigation.
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