Thyroid hair loss is usually diffuse: your hair thins everywhere rather than in one spot. Both an underactive and an overactive thyroid can disturb the hair cycle, pushing too many hairs into the shedding phase at once. In most cases it is temporary, and hair grows back once your values are stable.
What bothers me about the search results for this question: nearly everything you find comes from a hair clinic or a shampoo brand. Useful information, but with an interest attached.
One thing is missing as a result, and almost every woman with this complaint needs it: a timeline.
Can an underactive thyroid cause hair loss?
Yes. Thyroid hormone helps steer how fast hair follicles move through their growth cycle. With an underactive thyroid that cycle slows, so more hairs move into the resting phase together and shed afterwards. The same happens with an overactive thyroid, through a different mechanism.
The pattern that goes with it is called telogen effluvium: scattered shedding across the whole scalp, without bald patches. You notice it mainly in your brush, in the shower and in a thinner ponytail.
A large Turkish review of 2,851 women with telogen effluvium showed how often thyroid abnormalities and iron deficiency recur in this group (Karakoyun et al., 2025). In women it is rarely one cause.
Which is exactly why a thyroid test rarely stands alone here.
How do you recognise thyroid-related hair loss?
Thyroid-related hair loss is nearly always diffuse and comes with other symptoms. Think of feeling cold, fatigue, dry skin, more brittle nails, constipation and a changing cycle. Losing hair in defined patches, or only at the crown, points more towards another type of hair loss.
These points help tell them apart:
- Diffuse across the whole scalp points to a systemic cause such as your thyroid or your iron
- A receding hairline or thinning crown fits androgenetic hair loss better
- Round bald patches fit alopecia areata, a different autoimmune condition
- Shedding that starts three to four months after an event fits telogen effluvium
That three to four month delay is the important one. Your hair is not responding to today, it is responding to what happened in the spring.
Our article on hair loss in women sets the hormonal causes side by side, and postpartum hair loss covers the same delay after a pregnancy.
Why is my hair falling out since I started medication?
This is the most confusing experience on this subject. More shedding in the first months after starting levothyroxine happens regularly. Your hair cycle resets to the new hormone status, pushing a larger group of hairs into the shedding phase at once. It is usually temporary.
It feels like proof the medicine is wrong. Usually it is the opposite: your hair cycle is in the middle of resetting.
I mention this explicitly because it is why some women stop on their own at this stage. Do not do that without discussing it. Talk to your GP, who can look at the values alongside it. More on these effects is in levothyroxine side effects.
Will your hair grow back?
In most cases yes. With hair loss from a thyroid problem the follicle stays intact, so hair can grow back once your values are stable. It is slow though: hair grows roughly a centimetre a month, so visible recovery takes months rather than weeks.
| Period | What you usually notice |
|---|---|
| First 1 to 3 months after correction | Shedding may stay the same or briefly increase |
| 3 to 6 months | Shedding drops, short new hairs along the hairline |
| 6 to 12 months | Visibly more volume, the ponytail thickens again |
| After 12 months | Largely recovered in most women |
That table is the reason I wanted to write this article. Without a timeline, three months is not a waiting period but a failure, and that is exactly when women start doubting their medication.
If it continues unchanged after a year of stable values, something else is probably going on.
What if your thyroid values are fine and your hair still falls out?
Then ferritin is the first value to look at. Iron deficiency is the most common cause of diffuse hair loss in women, and your haemoglobin can be perfectly normal alongside it. Heavier periods make this pattern more likely again.
Make it concrete: your TSH is 1.6 mIU/L, your free T4 sits mid-range, and your ferritin is 12 micrograms per litre. That conversation is about iron, not about your thyroid.
Why low ferritin produces symptoms with a normal haemoglobin is explained in iron deficiency without anaemia. The link with your period is in iron deficiency and hair loss in women and in anaemia from heavy periods.
Other values that regularly play a part in diffuse hair loss: vitamin D, vitamin B12 and zinc. And in women around forty, perimenopause is almost always somewhere in the background.
Which blood values say something here?
For the thyroid part it comes down to TSH, free T4 and anti-TPO. TSH shows whether your thyroid is falling behind, free T4 how much hormone is available, and anti-TPO whether an autoimmune cause sits behind it. For the non-thyroid part, ferritin is the key addition.
Anti-TPO is more interesting here than it looks. The Whickham Survey found the risk of developing an underactive thyroid was highest in women with both raised antibodies and a raised TSH (Vanderpump et al., 1995). Antibodies alone are therefore not an explanation for today's symptoms.
Per marker you can read what the value represents on our pages for TSH, anti-TPO and ferritin. The full explanation is in thyroid values explained.
What to discuss with your GP
Take to that conversation when the shedding began, whether it is diffuse or patchy, and what happened three to four months before. Think of a pregnancy, surgery, a strict diet, an illness or a stretch of heavy stress. That delay is clinically relevant.
Ask as well whether your ferritin is known, not just your haemoglobin. In women that is the difference between being found and being missed.
If you want a baseline of your own, Thyroid Complete looks at TSH, free T4, free T3 and anti-TPO, and Iron Status at your iron stores. The background to this cluster is in our article on Hashimoto, and on the cycle we wrote the thyroid and your cycle.
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
References
- Karakoyun Ö, et al. Retrospective Review of 2851 Female Patients With Telogen Effluvium: A Single-Center Experience. Journal of Cosmetic Dermatology, 2025. PMID 39950230
- Vanderpump MP, et al. The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. Clinical Endocrinology (Oxford), 1995. PMID 7641412
- Ralli M, et al. Hashimoto's thyroiditis: An update on pathogenic mechanisms, diagnostic protocols, therapeutic strategies, and potential malignant transformation. Autoimmunity Reviews, 2020. PMID 32805423
- NHG guidance on thyroid conditions, Dutch College of General Practitioners
- Thuisarts.nl, patient information on hair loss
Author
Lunarahealth
Dr. Naimi, BIG-registered physician, oversees the medical standards behind our content and assessments. Read our medical policy
