My girlfriend started finding clumps of hair in the shower drain about eighteen months after her PCOS diagnosis.
She didn't mention it for weeks. I only found out because I noticed a hairbrush in the bin. She'd thrown it away because looking at how much hair was left in it made her feel sick. When I asked, she said something that stuck with me: "I can deal with the weight. I can deal with the periods. But my hair..."
She didn't finish the sentence. She didn't need to.
If you've read the other articles in this series (the iron piece, the PCOS guide, the fertility article), you already know her story in pieces. The iron infusion when her ferritin was 8. The PCOS diagnosis at 24. The fertility fears. But this was the symptom that affected her most psychologically. Not the most medically significant one, but the one she saw in the mirror every morning.
I think that disconnect matters. Clinically, hair thinning is a symptom. Emotionally, it's an identity crisis. The gap between those two realities is where a lot of people get stuck, spending money on products, supplements, and treatments without ever checking whether the cause is something a blood test could identify.
The hair care market in Australia is worth billions. The hair loss treatment industry is enormous and growing. Yet the most common treatable causes of hair loss (iron deficiency, thyroid dysfunction, and hormonal imbalance) are identifiable with basic blood work. That blood work costs a fraction of what most people spend on topical products that may never address the underlying problem.
This article covers the blood tests that matter when you're losing hair, what each one reveals, and how to figure out whether your hair loss is something fixable from the inside.
A note before we get into it
General information only. I'm not a doctor or a dermatologist. Hair loss has many causes, and blood tests are one part of the investigation. Some types of hair loss are genetic, autoimmune, or stress-related and won't show up on a blood panel.
If you're experiencing significant or sudden hair loss, see your GP or a dermatologist. This article focuses on the blood markers most commonly investigated, not the full clinical workup.
Why hair loss hits differently
I debated whether to include this section, but my girlfriend read the draft and said to leave it in. So here it is.
Hair loss occupies a unique psychological space. Unlike fatigue, which you can hide. Unlike weight changes, which you can rationalise. Unlike period irregularities, which are invisible to everyone else. Hair loss is visible. It's daily. It's in the mirror, in the shower, on the pillow, in the hairbrush. Every single day.
Research consistently shows that hair loss has a disproportionate impact on self-esteem, body image, and mental health. For women especially, thinning hair often conflicts with deeply ingrained cultural expectations around femininity and attractiveness.
And here's the frustrating part: many people experiencing hair loss don't get blood work done for months or even years. They try shampoos, supplements, scalp treatments, dietary changes. Often spending significant money. Before anyone suggests checking whether their iron is depleted, their thyroid is struggling, or their hormones are out of balance.
My girlfriend spent about $400 on hair products before someone checked her ferritin. Which was, at that point, 8 µg/L. The answer had been in her blood the whole time.
The three most common treatable causes
When clinicians investigate hair loss, they're typically looking at three primary systems. Blood tests can assess all three.
1. Iron Deficiency
How iron affects your hair
Iron is needed for hair follicle cell division. When iron stores are low, the body prioritises more critical functions (oxygen transport, immune function, energy production) and hair growth gets deprioritised. Some of the iron your body stores is actually held in hair follicles, and when stores deplete, the body borrows from these less essential reserves first.
The number your GP might miss
Your Full Blood Count can look completely normal while your ferritin is low enough to cause hair loss. As one clinical review puts it: "blood is more essential than hair, and the body will shed hair before red blood cell indices become abnormal." A GP who only orders an FBC and sees normal haemoglobin might tell you everything's fine, when your iron stores are actually depleted and your hair is paying the price.
Research has found that women with diffuse hair loss have significantly lower mean ferritin levels than women without hair loss. Some dermatologists and trichologists recommend ferritin levels of at least 70–80 µg/L for optimal hair health. That's well above the standard lab reference range lower limit of 15 µg/L.
Who's most affected
Menstruating women (especially with heavy periods), vegetarians and vegans, endurance athletes, regular blood donors, and women with PCOS who often have heavy or irregular bleeding. If any of these describe you and you're losing hair, ferritin is the first thing to check.
For a deep dive on iron testing, see our iron and ferritin guide.
Test it with Bloody Good:
2. Thyroid Dysfunction
How your thyroid affects your hair
Thyroid hormones directly regulate the hair follicle cycle, influencing both hair growth and pigmentation. Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can cause diffuse hair thinning. Roughly 15% of people with certain types of hair loss have an underlying thyroid condition.
What the thinning looks like
Thyroid-related hair loss tends to be diffuse, meaning it thins across the entire scalp rather than in patches. Hair may also become coarser, dryer, or more brittle. In some cases, eyebrow thinning (particularly the outer third) can be an early sign of hypothyroidism.
The overlap problem
Thyroid symptoms and iron deficiency symptoms overlap heavily: fatigue, cold sensitivity, weight changes, hair loss. They can also coexist. In some cases, low iron actually impairs thyroid function, creating a compound problem where both contribute to hair loss at the same time.
For more on thyroid testing, see our thyroid function test guide.
Test it with Bloody Good:
Product: Thyroid Function Test (TFT)
3. Androgen Excess
How androgens cause hair loss
Androgens (testosterone, DHT, or dihydrotestosterone) influence hair follicle miniaturisation. In genetically susceptible individuals, elevated androgens cause scalp follicles to produce progressively thinner, shorter hairs until the follicle eventually stops producing visible hair. This is the mechanism behind androgenetic alopecia, or pattern hair loss in both men and women.
In women
Androgen excess is a hallmark of PCOS. Women with PCOS may experience thinning on the crown and along the part line (female pattern hair loss), alongside other androgenic symptoms like acne and hirsutism. The irony, which my girlfriend finds both amusing and infuriating, is that PCOS can cause excess hair where you don't want it (face, body) while simultaneously causing hair loss where you do (scalp).
What to test
Total testosterone, free testosterone (or Free Androgen Index), and SHBG. In women with PCOS, SHBG is often low (driven by insulin resistance), which means more free testosterone is available to act on hair follicles. Even if total testosterone looks borderline.
In men
Androgenetic alopecia is the most common cause of hair loss and is primarily genetic. Testosterone levels in men with male pattern baldness are usually normal. It's the scalp follicle's sensitivity to DHT, not the circulating hormone level, that determines the pattern. Blood tests are less diagnostically useful in this context, though they can rule out other contributing factors.
For more on testosterone and SHBG, see the testosterone article or the PCOS guide.
Test it with Bloody Good:
Product: Testosterone Free/Total + SHBG
The full hair loss blood panel
If you're losing hair and want a thorough investigation, here's the panel most dermatologists and trichologists would consider:
Iron studies (including ferritin). The single most important test for hair loss investigation. Ferritin is the marker that matters most.
Full Blood Count (FBC). Assesses haemoglobin and red blood cell indices. Important for detecting anaemia, but remember: FBC can be normal even when iron is low enough to cause hair loss.
Thyroid Function Test (TSH, Free T4, Free T3). Screens for both hypothyroidism and hyperthyroidism.
Vitamin D. Research has found significantly lower vitamin D levels in people with diffuse hair loss compared to healthy controls. Vitamin D receptors are present in hair follicles and may play a role in the hair cycle.
Vitamin B12. B12 deficiency can contribute to hair changes, especially in vegans, vegetarians, and older adults.
Zinc. Zinc deficiency has been associated with telogen effluvium (excessive hair shedding) and alopecia areata. It's less commonly tested than iron or thyroid, but worth including if the standard panel doesn't explain the hair loss.
Testosterone (total and free) / SHBG. For women with signs of androgen excess (acne, hirsutism, irregular periods, scalp thinning). Less useful for men with pattern baldness unless other androgen-related symptoms are present.
CRP. Worth including if an inflammatory or autoimmune component is suspected (e.g., alopecia areata, scarring alopecia).
Types of hair loss, and which ones blood tests can help with
Not all hair loss is the same, and blood tests are more useful for some types than others.
Telogen effluvium (diffuse shedding): The most common type related to internal factors. Triggered by physiological stress, including iron deficiency, thyroid dysfunction, nutritional deficiency, illness, surgery, hormonal changes (including post-pregnancy), or extreme psychological stress. Hair enters the shedding phase prematurely, usually 2–3 months after the trigger. Blood tests are highly relevant here.
Androgenetic alopecia (pattern hair loss): Genetic, progressive, driven by androgens (DHT). In women, blood tests help because androgen excess from PCOS or other causes can accelerate the pattern. In men, blood tests are less diagnostic since the condition is primarily genetic, but they can rule out contributing factors.
Alopecia areata (patchy loss): Autoimmune. The immune system attacks hair follicles. Blood tests may check for associated autoimmune conditions (thyroid antibodies, ANA) but won't directly diagnose alopecia areata itself.
Scarring alopecia: Inflammatory destruction of follicles. Requires dermatological assessment and often biopsy. Blood tests may reveal systemic inflammation but aren't the primary diagnostic tool.
Traction alopecia: Caused by sustained tension on hair follicles (tight hairstyles, extensions). Not a blood test problem.
When hair loss is NOT a blood test problem
I want to be honest about this because not every hair loss story ends with a fixable blood result.
Genetic pattern baldness. Male pattern baldness (androgenetic alopecia) in men is overwhelmingly genetic. Testing testosterone won't change the diagnosis or the trajectory. In women, there's a stronger case for testing because PCOS and other hormonal causes can be identified and managed.
Stress-related shedding. Telogen effluvium from acute stress, grief, major life events, or illness will often resolve on its own once the trigger passes. Blood tests can confirm nothing else is contributing, which is useful. But the fix is time, not a supplement.
Medication side effects. Some medications (certain blood thinners, antidepressants, retinoids, beta-blockers, chemotherapy) cause hair loss as a side effect. This is a conversation with your prescribing doctor, not a blood test issue.
Age-related thinning. Hair density naturally decreases with age. Some degree of thinning is physiological. Blood tests can help ensure it's not being accelerated by a deficiency, but they can't reverse ageing.
If your blood work comes back clean and you're still losing hair, the next step is a dermatologist or trichologist who can assess your scalp, your hair cycle, and potentially take a biopsy if needed.
How to prepare for testing
Fast if you're including iron studies. Fasting for 8–12 hours gives the most accurate serum iron and ferritin readings. Water is fine.
Test in the morning. Most hormones and iron markers are most consistent in the morning.
Stop biotin supplements 48–72 hours before. Biotin (vitamin B7), commonly found in hair and nail supplements, can interfere with thyroid and some hormone immunoassays. If you're taking a hair supplement that contains biotin (and most do), stop it before testing.
Stop iron supplements 24–48 hours before. Otherwise your serum iron will be artificially inflated.
Don't test when acutely unwell. Illness can elevate ferritin (masking true deficiency) and affect thyroid and inflammatory markers.
Mention everything you're taking. Hair supplements, hormonal contraception, prescription medications. All can affect results.
Understanding your results
The hair loss investigation is about patterns, not a single number. Here's what to look for:
Ferritin below 30 µg/L: Likely contributing to hair loss, even if your FBC is normal. Many hair specialists prefer ferritin above 70 µg/L for optimal hair health.
TSH outside the normal range (0.4–4.0 mU/L): Investigate further with Free T4 and Free T3. Both hypo- and hyperthyroidism can cause hair loss.
Vitamin D below 50 nmol/L: Deficiency. May be contributing to hair cycle disruption. Supplementation is generally recommended.
Elevated free testosterone or low SHBG (in women): Suggests androgen excess. Consider PCOS investigation if not already diagnosed.
Low B12 (below 200 pmol/L): May contribute to hair changes, particularly alongside other deficiencies.
Low zinc: Associated with hair shedding. Less commonly tested but worth checking if the standard panel doesn't explain the hair loss.
Multiple deficiencies: Common. Iron deficiency, vitamin D deficiency, and thyroid dysfunction frequently coexist, and their effects on hair compound each other. Treating one while missing the others may produce an incomplete response.
Tests to consider through Bloody Good
Core hair loss panel
Iron Studies (Including Ferritin) — the most important single test
Full Blood Count (FBC) — haemoglobin and red blood cell health
Thyroid Function Test (TFT) — TSH, Free T4, Free T3
Vitamin D (25-OH) — commonly deficient in people experiencing hair loss
Extended panel
Vitamin B12 — neurological and hair health
Zinc Blood Test — trace mineral linked to hair shedding
Testosterone Free/Total + SHBG — for women with signs of androgen excess
High-Sensitivity CRP — if inflammatory hair loss is suspected
Folate — works alongside B12 and iron
If you want broad coverage
The Bloody Good Test covers 100 biomarkers including iron studies, thyroid, vitamin D, B12, FBC, cholesterol, liver, kidney, and more. If you're investigating hair loss as part of a broader health check, this covers the hair-relevant markers plus everything else in one blood draw.
What to do after testing
If iron is low: Start supplementation as recommended by your GP. Expect 3–6 months before you see meaningful hair regrowth. Iron stores take time to rebuild, and the hair cycle is slow. Retest ferritin at 3 months to track progress.
If thyroid is abnormal: Your GP will initiate treatment (typically levothyroxine for hypothyroidism). Hair improvement usually follows within 3–6 months of thyroid optimisation, though it can take longer.
If androgens are elevated: Discuss management options with your GP. For PCOS, this may include insulin-sensitising medication, hormonal contraception, or anti-androgen therapy depending on your goals and circumstances. Lifestyle interventions (particularly those targeting insulin resistance) can also improve the hormonal picture.
If vitamin D is low: Supplement as directed. Retest at 3 months. Hair improvement from vitamin D correction alone tends to be more modest than from iron or thyroid correction, but it contributes to the overall picture.
If everything's normal: Blood work has done its job. It's ruled out the most common treatable causes. The next step is a dermatologist or trichologist for a clinical assessment of your scalp and hair cycle. Pattern baldness, autoimmune causes, and scarring alopecias require clinical diagnosis, not blood work.
Give it time. Hair has a long growth cycle. Whatever the cause and whatever the treatment, visible improvement typically takes 3–6 months at minimum. Patience isn't optional. It's biological.
Explore more biomarkers
Browse the Bloody Good Biomarker Directory
General information only. This article is not medical advice and is not a substitute for care from a qualified health professional. If you have concerning symptoms or urgent health issues, seek medical attention promptly.