Can levothyroxine cause hair loss? It is one of the most common concerns raised by patients starting this widely prescribed NHS thyroid medication. The answer is nuanced: hair loss is listed as a possible adverse effect in UK Summaries of Product Characteristics, yet the underlying thyroid condition itself is frequently the true culprit. Understanding the difference between medication-related and disease-related hair shedding is essential before making any changes to treatment. This article examines the clinical evidence, explains how to identify the cause, and outlines practical steps to manage hair loss safely while taking levothyroxine.
Summary: Levothyroxine can cause temporary hair loss as a listed adverse effect, but the underlying thyroid condition is often the more likely cause of hair thinning.
- Hair loss (alopecia) is listed in UK Summaries of Product Characteristics for levothyroxine, with frequency classified as 'not known'.
- The mechanism is thought to involve telogen effluvium — a temporary shift of hair follicles into the shedding phase triggered by changes in thyroid hormone levels.
- Both hypothyroidism and hyperthyroidism independently cause diffuse hair thinning, meaning the thyroid condition itself is frequently responsible rather than the medication.
- NICE (NG145) recommends rechecking thyroid function tests 6–8 weeks after starting or adjusting levothyroxine to confirm optimal dosing.
- Iron deficiency (low ferritin) is a well-recognised, treatable contributor to hair shedding and should be investigated if thyroid function is within range.
- Levothyroxine should not be stopped without GP advice; untreated hypothyroidism carries serious cardiovascular and metabolic risks.
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Levothyroxine and Hair Loss: What the Evidence Shows
Hair loss is a listed adverse effect of levothyroxine in UK SmPCs, generally described as temporary and thought to result from telogen effluvium triggered by shifts in thyroid hormone levels.
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Levothyroxine is a synthetic form of thyroxine (T4), the hormone naturally produced by the thyroid gland. It is widely prescribed in the UK for hypothyroidism and is one of the most commonly dispensed medications through the NHS. A frequent concern among patients starting this treatment is whether levothyroxine itself can cause hair loss.
The honest answer is nuanced. Hair loss (alopecia) is listed as an adverse effect of levothyroxine in UK Summaries of Product Characteristics (SmPCs); it is generally described as usually temporary, with frequency listed as 'not known' (meaning it cannot be estimated from available data). The mechanism is not fully established, but it is thought to relate to the hair growth cycle. When thyroid hormone levels shift — even towards normal — hair follicles may temporarily enter the telogen (resting and shedding) phase. This phenomenon is known as telogen effluvium, a diffuse, non-scarring form of hair shedding. According to NHS guidance, telogen effluvium typically resolves within a few months once the underlying trigger is addressed.
It is important to note that hair loss occurring shortly after starting levothyroxine is not necessarily caused by the medication. The timing can be coincidental, as the underlying thyroid condition itself is a well-established cause of hair thinning (discussed in the next section). Clinical evidence does not firmly establish levothyroxine as a direct, long-term cause of hair loss when thyroid hormone levels are appropriately managed. If hair loss persists beyond six months or worsens significantly, this warrants further clinical review rather than automatic discontinuation of the medication.
| Cause of Hair Loss | Likely Trigger | Key Signs | Typical Onset | Recommended Action |
|---|---|---|---|---|
| Levothyroxine (telogen effluvium) | Shift in thyroid hormone levels triggering telogen phase | Diffuse shedding after starting or increasing dose | Weeks after dose initiation or change | Usually self-limiting; recheck TFTs at 6–8 weeks per NICE NG145 |
| Under-treated hypothyroidism | Low T3/T4 slowing hair follicle turnover | Elevated TSH, fatigue, weight gain, dry skin, brittle nails | May predate diagnosis by several months | GP review of TFTs; adjust levothyroxine dose if TSH elevated |
| Over-treatment (excess levothyroxine) | Suppressed TSH causing iatrogenic hyperthyroidism | Palpitations, anxiety, heat intolerance, suppressed TSH | After dose increase | GP review; reduce dose to bring TSH within reference range |
| Alopecia areata (autoimmune) | Associated autoimmune disease, e.g. Hashimoto's thyroiditis | Patchy hair loss; unrelated to levothyroxine therapy | Variable; independent of treatment | Refer to NHS dermatology; not resolved by adjusting levothyroxine |
| Iron deficiency | Low ferritin impairing hair follicle function | Diffuse shedding; TFTs normal; low ferritin on blood tests | Variable | Full blood count and ferritin; treat deficiency with GP guidance |
| Androgenetic alopecia | Genetic/hormonal pattern hair loss; coincidental | Patterned thinning; TFTs and ferritin normal | Gradual onset | Dermatology referral; topical minoxidil may be considered |
| Levothyroxine formulation change | Product switch altering bioavailability | Hair changes after switching brand or formulation | After product switch | Per MHRA 2021 guidance: prescribe consistent product; recheck TFTs |
Why Thyroid Conditions Themselves Can Cause Hair Thinning
Both hypothyroidism and hyperthyroidism independently disrupt the hair follicle cycle, causing diffuse thinning that often predates diagnosis and may be mistaken for a medication side effect.
Before attributing hair loss to levothyroxine, it is essential to understand that both hypothyroidism and hyperthyroidism are independently associated with hair thinning and shedding, as noted in NHS guidance on both conditions. The thyroid hormones T3 and T4 play a critical role in regulating the hair follicle cycle. When these hormones are deficient or excessive, the normal rhythm of hair growth is disrupted.
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In hypothyroidism — the condition levothyroxine is prescribed to treat — low thyroid hormone levels slow cellular turnover throughout the body, including in hair follicles. This can cause:
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Diffuse hair thinning across the scalp
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Dry, brittle hair texture
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Loss of the outer third of the eyebrows (a non-specific clinical feature, not diagnostic on its own)
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Generalised hair shedding that may precede diagnosis by several months
Because hypothyroidism often develops gradually, many patients have already experienced significant hair loss before they receive their diagnosis and begin treatment. This means that hair shedding noticed in the early weeks of taking levothyroxine may actually be the tail end of disease-related hair loss, rather than a medication side effect.
Autoimmune thyroid disease, such as Hashimoto's thyroiditis — the most common cause of hypothyroidism in the UK — is also associated with other autoimmune conditions, including alopecia areata, a condition causing patchy hair loss (as noted by NHS guidance on alopecia areata). This further complicates the picture, as hair loss in this context is unrelated to levothyroxine therapy. Understanding this distinction is key to avoiding unnecessary anxiety or premature changes to treatment.
How to Tell Whether Your Medication or Condition Is Responsible
Reviewing thyroid function tests (TSH and free T4) is the key first step; a persistently elevated TSH suggests under-treatment, while a suppressed TSH points to over-treatment as the likely cause.
Distinguishing between medication-related and disease-related hair loss can be challenging, but there are several clinical clues that can help guide the assessment. The most important step is to review your thyroid function tests (TFTs), which measure levels of TSH (thyroid-stimulating hormone) and free T4. These are routinely monitored by GPs in line with NICE guidance (NG145) for hypothyroidism management. Note that free T3 is not routinely measured when monitoring treated primary hypothyroidism in primary care.
NICE recommends rechecking TFTs approximately 6–8 weeks after initiating or adjusting a levothyroxine dose, and at least annually once levels are stable.
Signs that the thyroid condition may still be responsible:
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Hair loss began before starting levothyroxine
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TSH remains elevated, suggesting under-treatment
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Other hypothyroid symptoms persist (fatigue, weight gain, cold intolerance)
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Hair loss is accompanied by dry skin and brittle nails
Signs that levothyroxine dosage may need review:
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Hair loss began after starting or increasing the dose
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TSH is suppressed below the laboratory reference range, suggesting over-treatment
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Symptoms of hyperthyroidism are present (palpitations, anxiety, heat intolerance)
If thyroid function tests are within the normal reference range and hair loss continues, your GP may consider investigating other causes. These include iron deficiency (low ferritin), which is a well-recognised contributor to diffuse hair shedding, as well as other causes such as androgenetic alopecia. Vitamin D and vitamin B12 deficiency are sometimes investigated in this context, though the evidence linking them directly to hair loss is limited and variable; testing should be guided by clinical judgement. A full blood count and ferritin measurement are a reasonable starting point.
If you take high-dose biotin (vitamin B7) supplements, it is important to inform your GP and the laboratory before having thyroid blood tests, as biotin can interfere with certain immunoassay-based TFTs and produce misleading results (MHRA Drug Safety Update, 2017).
For persistent, unexplained, or complex hair loss, referral to NHS dermatology is the appropriate pathway. Urgent or expedited referral should be considered if there are red-flag features such as scarring of the scalp, rapid patchy loss, scalp redness, scaling, pustules, or pain and tenderness — as these may indicate conditions requiring prompt specialist assessment. It is advisable to contact your GP rather than stopping levothyroxine without medical supervision, as abrupt discontinuation can have serious consequences for thyroid health.
Managing Hair Loss While Taking Levothyroxine
Optimising TSH within the laboratory reference range, addressing nutritional deficiencies such as low ferritin, and avoiding unnecessary dose changes are the main management strategies.
For most patients, hair loss associated with starting levothyroxine is self-limiting and improves as thyroid hormone levels stabilise, typically within a few months of achieving a therapeutic dose. Patience is often the most important aspect of management during this period.
There are several practical steps that may support hair health during treatment:
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Ensure optimal thyroid control: Work with your GP to confirm your TSH is within the laboratory reference range. NICE (NG145) recommends aiming to keep TSH within the local laboratory reference range for most adults on levothyroxine, though individual targets may vary depending on clinical circumstances.
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Address nutritional deficiencies: Iron deficiency (identified by a low ferritin or full blood count) is a treatable and well-recognised contributor to hair shedding. Vitamin D and vitamin B12 may also be checked based on clinical context, though evidence for their role in hair loss is variable. Ask your GP about relevant blood tests.
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Be aware of biotin supplements: High-dose biotin can interfere with thyroid blood tests. Discuss any supplements you are taking with your GP before having TFTs.
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Gentle hair care: Avoid excessive heat styling, tight hairstyles, and harsh chemical treatments, which can worsen fragile hair.
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Avoid unnecessary dose changes: Altering your levothyroxine dose without medical advice can destabilise thyroid function and potentially worsen hair loss.
Regarding levothyroxine brands and formulations: there is no robust evidence that switching products resolves hair loss. However, the MHRA has issued guidance (Drug Safety Update, May 2021) advising that if a patient develops symptoms — including hair changes — after switching between levothyroxine products, clinicians should consider prescribing a consistent product and rechecking thyroid function tests. If you suspect a particular formulation is causing problems, discuss this with your GP or pharmacist rather than switching independently.
In cases where hair loss is confirmed to be androgenetic or autoimmune in origin, separate treatments such as topical minoxidil or dermatology-led interventions may be considered. These decisions should always be made in consultation with a healthcare professional.
MHRA Guidance and Reporting Side Effects in the UK
Suspected hair loss from levothyroxine should be reported via the MHRA Yellow Card scheme; patients should not stop the medication without GP advice due to serious risks of untreated hypothyroidism.
In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) is responsible for monitoring the safety of medicines, including levothyroxine. Hair loss is acknowledged within the SmPCs for levothyroxine preparations available in the UK, and the MHRA continues to review post-marketing safety data as part of its ongoing pharmacovigilance activities.
If you believe you are experiencing hair loss as a side effect of levothyroxine, you are encouraged to report this through the Yellow Card scheme — the UK's system for reporting suspected adverse drug reactions. Reports can be submitted by both patients and healthcare professionals via the MHRA website (yellowcard.mhra.gov.uk) or through the Yellow Card app. These reports are valuable because they contribute to the national evidence base and can prompt regulatory review if a pattern of concern emerges.
When reporting, it is helpful to include:
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The brand and dose of levothyroxine you are taking
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When the hair loss started in relation to beginning or changing your dose
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Any other medications or supplements you are taking
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Relevant medical history, including recent thyroid function test results
It is important to emphasise that you should not stop taking levothyroxine without speaking to your GP first. Untreated or under-treated hypothyroidism carries significant health risks, including cardiovascular complications and, in severe cases, myxoedema coma. If hair loss is causing significant distress, a GP appointment is the appropriate first step — they can review your thyroid function, explore alternative causes, and refer you to a specialist if needed.
Frequently Asked Questions
How long does hair loss from levothyroxine last?
Hair loss linked to levothyroxine is generally temporary and typically resolves within a few months once thyroid hormone levels stabilise on a therapeutic dose. If shedding continues beyond six months or worsens, you should speak to your GP to rule out other causes such as iron deficiency or androgenetic alopecia.
Can taking too much or too little levothyroxine make hair loss worse?
Yes — both over-treatment and under-treatment with levothyroxine can contribute to hair shedding. A suppressed TSH suggests the dose may be too high, mimicking hyperthyroidism, while an elevated TSH indicates under-treatment; both states disrupt the hair follicle cycle. Your GP can adjust your dose based on thyroid function test results.
Is hair loss a common side effect of levothyroxine?
Hair loss is listed as an adverse effect of levothyroxine in UK product information, but its frequency is classified as 'not known', meaning it cannot be reliably estimated from available data. Many cases of hair loss in people taking levothyroxine are actually caused by the underlying thyroid condition rather than the medication itself.
What is the difference between hair loss caused by hypothyroidism and hair loss caused by levothyroxine?
Hypothyroidism-related hair loss typically begins before or around the time of diagnosis, when thyroid hormone levels are low, and often improves as treatment takes effect. Levothyroxine-related hair loss, if it occurs, usually starts after beginning or changing the dose and is thought to be a temporary telogen effluvium triggered by the shift in hormone levels.
Should I stop taking levothyroxine if I notice hair loss?
No — you should not stop levothyroxine without speaking to your GP first, as untreated hypothyroidism carries serious health risks including cardiovascular complications. Book a GP appointment to review your thyroid function tests and explore whether the hair loss has another treatable cause, such as iron deficiency.
Can switching between different brands of levothyroxine affect hair loss?
There is no robust evidence that a specific levothyroxine brand causes hair loss, but MHRA guidance advises that if symptoms — including hair changes — develop after switching products, clinicians should consider prescribing a consistent formulation and rechecking thyroid function. Discuss any concerns about your formulation with your GP or pharmacist rather than switching independently.
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