Hair Loss With an Underactive Thyroid, Even on Medication

Hair can keep thinning on thyroid medication for several reasons, and most of them are fixable or temporary. Shedding often lags two to four months behind whatever set it off, including the months before your thyroid was treated or a recent dose change. Other common causes are a dose that is not quite right yet (too low or too high), low iron, and pattern hair loss, which is the most common kind of all and has nothing to do with your thyroid. Patchy hair loss is different again and worth showing a doctor.

Finding hair in the brush or the shower drain when your results say "normal" is upsetting, and very common. This guide walks through the likely causes so you can work out what to ask your doctor.

Why the thyroid affects your hair

Hair follicles respond directly to thyroid hormone. Laboratory research on human scalp follicles shows thyroid hormone helps keep them in their growth phase. That is why an underactive thyroid can cause diffuse thinning, and why the Australian consumer medicine information for levothyroxine lists hair loss among the symptoms of an underactive thyroid.

The delay: shedding that shows up months later

The most common pattern is called telogen effluvium. A shock to the body, such as illness, surgery, a big stress, crash dieting, or a thyroid that has been out of balance, pushes many hairs into their resting phase at once. They fall out a few months later. The Australasian College of Dermatologists says the shedding is usually noticed three to four months after the trigger, and other sources put it at two to three.

That delay is why hair often keeps falling out after your thyroid levels have settled: it is the echo of a trigger from months before. This kind of shedding is usually temporary. It can last three to twelve months, and in most cases hair regrows. The College also notes that no medical treatment speeds the regrowth, which is worth remembering whenever a product promises to.

Your dose: too little or too much

Hair can thin when thyroid hormone is too low, and also when it is too high. The Australian prescribing information for levothyroxine lists hair loss among the signs of a replacement dose that is too high. The consumer information also lists hair loss among the less serious side effects of the medicine.

Never change your dose yourself. Ask your doctor what your latest TSH and free T4 were, whether your dose has changed recently, and whether your levels are where they want them to be. If a dose change could be a trigger, the shedding may simply need time.

Iron and other nutrients

Low iron is common alongside an underactive thyroid and Hashimoto's, and it is often raised as a cause of hair shedding. The research is genuinely mixed: some studies find a link, others do not, and there is no agreed ferritin level for hair. It is still worth having iron checked, because low iron causes plenty of other problems, including tiredness. Our guide to iron, ferritin and the thyroid hair loss nobody connects goes into detail.

For other nutrients the rule is "test, don't guess". Dermatology reviews agree that genuine deficiencies should be corrected, but there is little evidence that extra vitamins or minerals help hair when you are not deficient, and some can make things worse. Biotin is the classic example. There are no clinical trials showing it helps hair loss in people who are not deficient, and high-dose biotin can make thyroid test results read wrongly. If you take it, tell whoever orders your tests.

Pattern hair loss: the most common cause of all

Pattern hair loss (androgenetic alopecia) is the most common form of hair loss overall, and it becomes more common with age. It often shows up as gradual thinning over the top of the head or a widening part. It can happen at the same time as thyroid-related shedding, so one can unmask the other. The Australasian College of Dermatologists notes that it is not caused by diet, so it is worth getting it diagnosed rather than trying to fix it with supplements. There are effective treatments, and a GP or dermatologist can talk you through them.

Patchy hair loss is different

Round, smooth bald patches are usually alopecia areata, an autoimmune condition. It is linked to thyroid autoimmunity: people with alopecia areata are more likely to have thyroid antibodies or thyroid disease. It is not the same as thyroid-related thinning and needs its own assessment, so show your doctor or a dermatologist.

When to see a doctor

• Losing around 100 hairs a day is normal. If you are losing clearly more, or it comes out in clumps, talk to your doctor.
• See someone promptly for patchy bald spots, a sore, itchy or burning scalp, or rapid hair loss with no obvious trigger.
• If you have started, stopped or changed any medicine in the past few months, mention it.

Questions to ask your doctor

1. What were my latest TSH and free T4, and are they where you want them?
2. Could a recent dose change, illness or stress explain this shedding?
3. Can we check my iron and ferritin, and anything else that fits my symptoms?
4. Does this look like shedding, pattern hair loss, or something else?
5. Should I see a dermatologist?
6. Is anything I take, such as biotin, affecting my test results?

Where ThyroBase fits

We will be straight with you: no nutrition product can regrow hair, and that includes ours. ThyroBase is daily nutrition built for life with an underactive thyroid or none at all. It has no added biotin, so it will not skew your thyroid tests, and it is designed to sit alongside your thyroid medication, never instead of it. For hair, the most useful steps are the ones above: the right dose, checked iron, a proper diagnosis, and time. Here is who ThyroBase is, and is not, designed for.

Hair is slow to tell you things have changed.

It usually shows the past few months, not today. Give it time once the cause is sorted.

Frequently asked questions

Why is my hair still falling out on levothyroxine?

Common reasons include delayed shedding from a trigger a few months earlier (including the time before treatment or a dose change), a dose that is not quite right yet, low iron, and pattern hair loss, which is common and unrelated to the thyroid. Ask your doctor to check your TSH, free T4 and iron, and whether a dermatologist could help.

Can levothyroxine cause hair loss?

Hair loss is listed among the less serious side effects in the Australian consumer medicine information for levothyroxine, and the prescribing information lists hair loss as a sign of a dose that is too high. Never change your dose yourself. Talk to your doctor if you are worried.

Will my hair grow back once my thyroid is treated?

Shedding triggered by an out-of-balance thyroid or another shock is usually temporary, and in most cases hair regrows. It can take months, because hair grows slowly. Pattern hair loss and patchy hair loss are different conditions and need their own diagnosis.

Do supplements help thyroid hair loss?

Only if you have a real deficiency, such as low iron, confirmed by a test. There is little evidence that extra vitamins help hair otherwise, and high-dose biotin can make thyroid test results read wrongly. Ask your doctor to test before you buy.

The honest takeaway

Hair loss on thyroid medication is common, often temporary, and usually has a findable cause. Remember the two-to-four-month delay, ask whether your dose is right, get your iron checked, and have pattern or patchy hair loss properly diagnosed. Be wary of any product that promises regrowth. Then give your hair the time it needs.

ThyroBase is a nutritional supplement and is not intended to diagnose, treat, cure, or prevent any disease. It is designed to sit alongside your thyroid medication, not replace it. This article is general information, not medical advice. Never start, stop or change a supplement or prescription medication without speaking to your doctor or pharmacist first.

References

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Australasian College of Dermatologists. A-Z of Skin: Telogen effluvium. dermcoll.edu.au.
healthdirect Australia. Alopecia (hair loss). healthdirect.gov.au. Last reviewed August 2024.
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Australasian College of Dermatologists. A-Z of Skin: Female pattern hair loss. dermcoll.edu.au.
Xin C, Sun X, Lu L, et al. Increased incidence of thyroid disease in patients with alopecia areata: a systematic review and meta-analysis. Dermatology. 2020;236(3):251-4.