Blood tests for hair loss: what they can reveal
Reviewed by the My London Pharmacy clinical team, Trishla Shah, Pharmacist Independent Prescriber
·17 August 2026
·6 min read
Quick answer
Blood tests are useful for hair loss when a physical cause is suspected, commonly checked markers include ferritin (iron stores), thyroid function, and hormone levels such as testosterone or DHEA-S where excess hair loss with other symptoms suggests a hormonal cause. They are less useful for typical male or female pattern hair loss, which is usually diagnosed by pattern and history rather than blood work.
When blood tests genuinely help with hair loss
Hair loss has several distinct causes, and blood tests are useful for some of them and largely unnecessary for others. They earn their place when the pattern of shedding, its timing, or accompanying symptoms suggest an underlying physical cause rather than the common, largely genetic pattern hair loss that affects a large share of both men and women over time.
The clues that point towards testing being worthwhile: sudden or unusually heavy shedding rather than gradual thinning, shedding all over the scalp rather than a typical pattern, or hair loss alongside other symptoms, fatigue, weight change, irregular periods, or skin and nail changes. Your clinician's assessment of your history and the pattern of loss is what decides whether testing adds anything.
Iron and ferritin
Low iron stores are one of the best-established links with hair shedding, particularly in women. Ferritin can affect the hair growth cycle even before it drops low enough to cause anaemia, which is why ferritin is often checked specifically, not just a general full blood count. This is particularly relevant after heavy periods, pregnancy, significant blood loss, or with a diet low in iron-rich foods.
Correcting a genuine iron deficiency does not regrow hair overnight, the hair cycle is slow, so it typically takes several months of normalised levels before shedding visibly improves, but identifying and correcting it removes a real, ongoing driver of loss.
Thyroid function
Both an underactive and an overactive thyroid can cause diffuse hair thinning, usually alongside other symptoms such as fatigue, weight change, or feeling unusually cold or warm, see our guide on how often to check your thyroid for more on testing frequency. Thyroid-related hair loss tends to be diffuse rather than patterned, and it typically improves once thyroid levels are properly treated and stable, though again this takes months rather than weeks.
Hormone levels
For women, hair thinning alongside irregular periods, acne, or unwanted hair growth elsewhere can point towards a hormonal cause, and testing may include markers such as testosterone, DHEA-S or other androgens, our guide to unwanted hair growth covers the related picture of excess androgen effects. In postpartum hair loss, the trigger is usually the sharp hormone shift after childbirth rather than an ongoing hormonal problem, and it is common enough to be considered normal, our guide to postpartum hair loss explains what to expect and when it is worth a closer look.
For men with pattern hair loss, hormone testing is rarely useful, since male pattern hair loss is driven by genetic sensitivity to normal hormone levels rather than an excess of them, the pattern and family history usually tell the story on their own.
Other markers worth knowing about
Vitamin D and zinc deficiency are both linked with hair shedding, though the evidence is less clear-cut than for iron or thyroid function, and correcting a genuine deficiency is more likely to help than supplementing without testing first. In persistent or unexplained cases, a wider panel may be considered to rule out less common causes, guided by your specific history rather than tested as routine.
What blood tests generally cannot tell you is whether you have male or female pattern hair loss, that diagnosis rests on examining the pattern of thinning, your family history and how it has progressed over time, not on a laboratory number.
Stress, medication and other triggers
Beyond the markers a blood test measures directly, a few other causes of hair shedding are worth knowing, since they influence how a clinician interprets your results. Telogen effluvium, a temporary, diffuse shedding roughly two to three months after a triggering event, commonly follows significant physical stress: a high fever or illness, surgery, rapid weight loss, or a major life stressor. Blood tests in this situation are often normal, because the cause is a stress response rather than a nutrient or hormone problem, and the reassurance that levels are normal, alongside the typical timeline, is itself part of the diagnosis.
Certain medications are also recognised causes of hair shedding, including some blood pressure medicines, antidepressants, retinoids and hormonal treatments, if you have started a new medicine in the months before shedding began, it is worth mentioning specifically, since blood tests will not identify a drug-related cause on their own.
What to expect from the process
A typical work-up starts with your clinician taking a detailed history, when the shedding started, whether it is patterned or diffuse, family history, recent illness, weight change, diet, medications, and for women, menstrual and reproductive history. This history often does more to narrow down the likely cause than the blood test itself, and it is what determines which markers are actually worth checking rather than ordering everything indiscriminately.
Results are usually available within a few days, and your clinician interprets them alongside your history rather than in isolation, a borderline ferritin result means something different in someone with heavy periods and gradual shedding than in someone with sudden, patchy hair loss and no other symptoms. If a deficiency or hormonal imbalance is found, treatment addresses that directly; if nothing shows up, that itself is useful information that shifts the conversation towards pattern hair loss and its own treatment options, or towards other causes worth investigating further.
Getting tested
If your hair loss is sudden, diffuse, or accompanied by other symptoms, testing is a reasonable and often clarifying first step, and if a cause is found, addressing it is usually more effective than any topical or oral treatment aimed at the hair alone. If the pattern points more towards typical pattern hair loss, your time may be better spent discussing treatment options directly, which our hair loss treatment guide covers in full.
Our clinical team can advise on whether testing makes sense for your specific pattern of hair loss, and offers blood testing with results reviewed by a clinician so any findings come with a clear next step rather than a number on its own.
Frequently asked questions
Should I get blood tests for hair loss?
It depends on the pattern. Sudden, diffuse, or all-over shedding, or hair loss with other symptoms, is worth testing. Typical gradual pattern hair loss is usually diagnosed by history and pattern rather than blood work.
What is the most useful blood test for hair shedding?
Ferritin (iron stores) is one of the most consistently useful markers, particularly in women, alongside thyroid function tests.
Can low vitamin D cause hair loss?
There is a link, though the evidence is less clear-cut than for iron or thyroid problems. Correcting a genuine deficiency is more likely to help than supplementing without testing.
Do blood tests diagnose male or female pattern hair loss?
No. Pattern hair loss is diagnosed by the pattern of thinning, progression over time and family history, not by a blood test.
How long after correcting a deficiency does hair loss improve?
Typically several months. The hair growth cycle is slow, so improvement lags behind the blood test returning to normal.