Hair loss has many possible causes, and not all of them are visible from the pattern of shedding alone. A blood test will not diagnose hair loss on its own, but it can rule in or rule out several treatable conditions that either cause shedding directly or make an existing pattern worse. Knowing what these tests look for, and what a result actually means, helps you understand what a clinician is checking for and why testing is sometimes skipped altogether.
This article sets out the blood tests commonly used to investigate hair loss in the UK, what each one is looking for, how to interpret the results, an important warning about a common supplement that can distort thyroid results, and what happens after testing is complete.
Why test before treating
Hair loss broadly falls into two categories: a hormonally driven pattern such as androgenetic alopecia, which follows a recognisable distribution and is usually diagnosed by examination alone, and diffuse or unexplained shedding, known as telogen effluvium, which is more likely to have an underlying medical trigger worth identifying. Blood testing is most useful in the second group, where a nutrient deficiency, thyroid disorder or hormonal imbalance could be contributing.
Testing before starting treatment matters for two reasons. First, it can reveal a cause that needs its own management, such as an underactive thyroid, where treating the underlying condition is likely to help hair recover without any hair-specific treatment at all. Second, it avoids months of trial and error with supplements or medications aimed at a cause that was never actually present. A clinician will use your history alongside any test results to decide what, if anything, needs treating.
Not every case needs testing, and it is worth being upfront about that. Typical male-pattern hair loss, receding at the temples and thinning on the crown in a man with no other symptoms, is usually diagnosed by the pattern alone, and blood tests rarely change the plan. Testing earns its place when the story does not fit that mould: shedding that is diffuse rather than patterned, a sudden increase in hair in the shower or on the pillow, recent illness, significant weight change, a new medication, or other symptoms such as fatigue, irregular periods or skin changes alongside the hair loss.
Ferritin, thyroid, vitamin D and hormones
A full blood count is usually the starting point, picking up anaemia or other blood abnormalities that can contribute to shedding. Ferritin, the storage form of iron, is checked alongside it because low iron stores have been linked to increased hair shedding, particularly in women, even before iron-deficiency anaemia develops on the full blood count.
Thyroid function tests (TSH, and free T4 where indicated) are a standard part of the panel, since both an underactive and an overactive thyroid are recognised causes of diffuse hair loss. Vitamin D is commonly checked too; deficiency is widespread in the UK, particularly over autumn and winter, and low levels have been associated with several forms of hair loss in observational studies. B12 and folate are added when diet or symptoms suggest a reason to check them, such as a strict vegan diet or a malabsorption condition.
Where the pattern of hair loss suggests a hormonal cause, for example irregular periods, acne or other signs of androgen excess in women, a clinician may add testosterone, SHBG (sex hormone-binding globulin) and sometimes DHEA-S to look for an underlying condition such as polycystic ovary syndrome. In men, testosterone is not routinely needed to diagnose typical male-pattern hair loss, since that pattern is driven by scalp sensitivity to DHT rather than the absolute testosterone level; our guide to whether low testosterone actually causes hair loss covers this distinction in more detail.
Interpreting results
A result that falls within the standard laboratory reference range is not automatically "normal enough" for hair. Ferritin is a good example: a level toward the bottom of the normal range in someone with unexplained diffuse shedding is often still worth discussing with a clinician, even though it would not be flagged as abnormal on a routine report. Reference ranges are built for population-wide disease screening, not specifically for hair growth, so context from your symptoms and history matters as much as the number itself.
Thyroid results are usually more clear-cut: a TSH outside the normal range points toward an underactive or overactive thyroid that needs its own management, and hair often improves once thyroid levels are corrected, though this can take several months. A single abnormal result should generally be discussed with a clinician and, where appropriate, repeated before starting treatment, since values can fluctuate and testing conditions (timing, recent illness, medication) can all affect the reading.
It is also worth remembering that a completely normal panel does not rule out hair loss having a cause; it simply means the tested causes are not the explanation this time. Many cases of androgenetic alopecia produce entirely normal bloods, because the mechanism is local scalp sensitivity to hormones rather than a systemic abnormality that a blood test would detect.
Timing of the shedding relative to the trigger is another piece clinicians weigh alongside the numbers. Hair sheds in response to what was happening in the body two to three months earlier, so a stressful illness, a crash diet, surgery or childbirth can all explain an episode of shedding that has nothing to do with today's blood results. This delay is part of why a single snapshot test cannot always give a complete answer, and why a clinician taking a history is what turns a set of numbers into a useful explanation rather than a checklist.
Biotin interference warning
One of the most important practical points before any hair-related blood test is biotin. High-dose biotin supplements, widely sold as "hair, skin and nails" formulas, can interfere with several immunoassay-based blood tests, including thyroid function tests and some hormone assays. This can produce a falsely abnormal or falsely normal result, depending on the assay, at exactly the point you are trying to get a reliable answer.
Most UK laboratories and clinics advise stopping biotin supplements for at least two to three days before a blood test, and some recommend longer for very high-dose products. If you are taking a hair supplement and are not sure whether it contains biotin, checking the label or asking your pharmacist before your appointment is a sensible precaution. If a result looks unexpectedly abnormal and you have been taking biotin, mention this to whoever is interpreting it, since a repeat test after stopping the supplement is often the simplest way to resolve the uncertainty.
Treatment routes after testing
What happens next depends on what testing finds. A confirmed iron or vitamin D deficiency is usually treated with supplementation and rechecked after a few months, since hair growth is slow and improvement is not immediate. A thyroid abnormality is referred for its own management, typically by a GP, with hair recovery expected to follow once levels are stable.
Where testing is normal and the pattern is consistent with androgenetic alopecia, licensed treatments such as finasteride and minoxidil are the options with the strongest evidence behind them, and neither requires abnormal bloods to be appropriate. A clinician will discuss suitability, likely timelines and monitoring with you individually before starting either.
Where a hormonal cause such as suspected polycystic ovary syndrome is identified in a woman, treatment addresses the underlying hormonal pattern as well as the hair itself, and may involve a different specialist. Our broader guide to hair loss in women, causes and treatment options covers these pathways in more depth, and our review of the evidence for vitamins and hair loss looks specifically at when supplementing actually helps.
Whatever the result, the same principle applies: testing turns "might something be causing this" into a specific, answerable question, and treatment is most effective when it targets whatever the answer turns out to be, rather than a guess made without it. Your clinician will advise based on your individual circumstances and the results of any testing.