Thyroid problems and hair loss

Your hair has been falling out for weeks. Someone tells you to get your thyroid checked. Or the other way round: you’ve just been diagnosed with a thyroid problem, you start treatment, and that’s exactly when your hair starts falling out. Either way, the question is the same: is the thyroid to blame, and for how long?

Women are especially affected. In the UK, the most common cause of an underactive thyroid, Hashimoto’s disease, is more common in women and usually starts between the ages of 30 and 50¹.

How the thyroid makes hair fall out

The thyroid is a small gland at the base of your neck. Whether it works too hard (hyperthyroidism) or not hard enough (hypothyroidism), it can cause hair loss².

It usually takes a severe, long-lasting imbalance. The British Thyroid Foundation (BTF) explains that the loss then affects the whole scalp, without clear patches, and that it’s unusual for a mild problem to make your hair fall out³. Your eyebrows can thin too; in most cases, it’s temporary³.

The mechanism is similar to other sudden shedding. A thyroid problem is one of the triggers of telogen effluvium: lots of hairs enter the resting phase at the same time, then fall out together⁴.

The other signs

Thyroid-related hair loss with no other sign at all is unusual³. These symptoms can point you in the right direction.

Underactive thyroid: extreme tiredness, feeling cold, weight gain, constipation, trouble concentrating, low mood, dry skin and hair, a hoarse voice, irregular or heavy periods¹. These signs often creep in slowly, and they’re easy to put down to everyday life.

Overactive thyroid: nervousness, irritability, trouble sleeping, tiredness, not coping with heat, sweating, a fast or irregular heartbeat, shaking, weight loss despite a good appetite. The NHS also mentions hair thinning or falling out in places⁵.

Many of these signs have other possible causes. A blood test is usually what sorts it out⁵.

Why the shedding comes late

A hair that enters the resting phase doesn’t fall out right away. In telogen effluvium, the shedding becomes noticeable 2 to 4 months after the trigger⁴. For the thyroid, the BTF puts it simply: hair loss becomes visible several months after the problem begins³.

This gap sets a trap. The problem is found, treatment starts, and a few weeks later your hair falls out. You blame the treatment. The BTF warns that treatment sometimes gets the blame unfairly, and that stopping it makes the hair loss worse³. Treatments for an overactive thyroid can, in rare cases, cause hair loss, but according to the BTF they’re very unlikely to be the cause³.

If in doubt, don’t stop anything on your own: talk to your doctor, who can adjust the treatment if needed.

Is it really the thyroid?

Not always. A 2023 literature review reports that a thyroid abnormality is found in 5.7% to 17% of telogen effluvium cases, depending on the study⁶. That’s not rare, but it means most of these cases have another cause.

A Turkish study points the same way. It compared 90 women with chronic telogen effluvium to 90 women with no shedding: their thyroid results were no different⁷. The sample is small and comes from a single hospital, but the authors conclude that deficiencies are less common than thought in this kind of hair loss and that other causes should be looked for⁷.

So your doctor will check the rest too. The BTF lists low iron and polycystic ovary syndrome among the causes of diffuse hair loss³. The French Society of Dermatology adds low vitamin D and B12². We go through these causes in our article on hair loss in women.

Blood tests and biotin

Many hair supplements contain biotin, a B vitamin. In high doses, it can skew thyroid test results and make it look as if your thyroid is overactive³.

British Thyroid Foundation³

The BTF advises stopping biotin at least two days before a blood test³. Tell your doctor if you take it: they’ll tell you when to stop.

After giving birth

Hair loss in the months after a baby is born is very common and temporary. We explain it in our article on postpartum hair loss. But the thyroid can also go off balance at that time.

Postpartum thyroiditis is a temporary inflammation of the thyroid. It happens in fewer than one pregnancy in ten, usually within twelve months of the birth⁸. It often starts with a phase where the thyroid goes into overdrive (weight loss, anxiety, not coping with heat, shaking), followed by a phase where it slows down (tiredness, low mood, feeling cold)⁸. If your hair loss comes with these signs, a blood test can check your thyroid².

Round patches: alopecia areata

A thyroid problem makes hair fall out all over the head. Round or oval patches that are smooth and bald are something else: alopecia areata². It’s an autoimmune disease, where the immune system attacks the hair follicles. In most cases, it’s temporary³.

It’s more common in people who have an autoimmune thyroid disease³. A meta-analysis of 17 studies, covering 2,850 people with alopecia areata and 4,667 controls, found thyroid disease much more often in alopecia areata, with odds about 3.7 times higher⁹. That’s an association, and it doesn’t tell you which one causes the other. The authors recommend checking the thyroid in people with alopecia areata⁹.

When the hair grows back

Once the problem is properly treated, the hair usually grows back, but it takes several months and may not be complete³. The American Academy of Dermatology agrees: treating thyroid disease can reverse the hair loss¹⁰. The new hair sometimes has a slightly different texture or color³.

For telogen effluvium in general, the shedding gradually eases over 6 to 9 months in most cases⁴. Count in seasons, not weeks.

When to see a doctor

Tracking regrowth with photos

With the thyroid, everything is slow. The shedding comes months after the problem, the regrowth months after the treatment. In between, it’s very hard to tell if things are improving by looking in the mirror, under different light every day.

Take the same photo every week instead: same room, same light, hair dry and styled the same way. Aim at the part and the crown, where volume shows best, and take one of your hairline for the short new hairs at the temples.

That’s what Calvap does. Your photos stay on your phone and line up on a timeline. The journal lets you note the date of your diagnosis, treatment changes agreed with your doctor, and your blood tests. After a few months, you can match the trend to those dates. Only a blood test tells you whether your thyroid is under control. Your photos show what your hair is doing in the meantime.