Sapiens · Thyroid & Hair

Thyroid and Hair Loss: Why It Happens and Whether It Grows Back

If you are pulling hair out of the shower drain and your thyroid has just come back abnormal, the two are probably connected. Thyroid shedding is diffuse across the whole scalp, it starts two to three months after the thyroid problem itself, and in most people it reverses once your levels are corrected.

That delay confuses almost everyone, including some doctors. It also means the shedding you are watching this week is really a report on where your thyroid was two months ago, which changes how you read it and how patient you need to be with the tablet you have just started.

Is it the thyroid, or is it something else? Copy link

Start here, because this is where most people go wrong. Hair falling out is a symptom with a long list of causes behind it, and your thyroid is only one entry on that list, sitting alongside iron, illness, childbirth, medication and plain genetics. The pattern tells you far more than the amount does.

What thyroid shedding looks like. Hair comes out evenly from all over your scalp. Your parting may widen and your ponytail gets thinner, but you do not develop a bald patch. It comes out by the root, often in the shower or on the pillow. You notice it as volume gone, not a specific area gone bare. This is telogen effluvium, and it means more of your follicles than usual have switched into the resting phase at once.

What it does not look like. If your hairline is creeping back at the temples, that is pattern hair loss. Same if you are thinning at the crown while the sides stay dense. Neither has anything to do with your thyroid. If you have a smooth, coin-sized bald circle with a sharp edge, that is alopecia areata. That one does travel with thyroid disease, since autoimmune thyroid disease raises the risk of other autoimmune conditions, but it is a separate problem and needs different treatment.³

Naturally, you can have both at once, and plenty of people do. A thyroid problem can unmask pattern hair loss that was already creeping along quietly, which is why the shedding sometimes does not fully resolve even after your levels are fixed.

Why the thyroid does this to hair Copy link

Your hair follicles cycle. Most are growing at any given time, a small share are resting, and the resting ones eventually let go so a new hair can push through. Thyroid hormone is one of the signals that sets the length of the growing phase, and your follicles carry receptors for it directly.⁵

When thyroid hormone drops, the growing phase shortens and a larger batch of follicles slips quietly into the resting phase together, at roughly the same time, which is the entire mechanism in one sentence. They do not fall immediately. They sit there for roughly two to three months, still attached, and then release more or less at once. That is why the shedding arrives so long after the thyroid trouble started, and why it can appear just as you are starting to feel better on treatment.

Of course, this cuts the other way too. When you correct the thyroid, the follicles have to finish their resting phase and then start growing again before you see anything. The improvement lags just as the shedding did.

How common is this, actually? Copy link

Common enough to test for, and nowhere near universal. In a study of 460 consecutive hypothyroid patients in Kashmir, roughly four in ten reported hair loss or were found to have it on examination.¹ One hospital series, mostly women, so read it as a working figure and not a national rate. Thyroid disease itself is common here. A widely cited review projects that about 42 million people in India are affected,⁶ though that is a projection built from several separate studies and not a single national measurement.

Both directions of thyroid trouble can do it. An underactive thyroid is the more common cause simply because hypothyroidism is more common, and it tends to bring coarse, dry, brittle hair along with the shedding. An overactive thyroid produces a finer, softer texture and hair that feels limp. A retrospective study of women with telogen effluvium found significantly higher TSH in the hypothyroid group, while the hyperthyroid group did not differ significantly from women with normal thyroid function.² Retrospective, single centre, women only. Treat it as one data point.

What about the eyebrow sign? Copy link

Losing the outer third of your eyebrows is the classic teaching sign of an underactive thyroid. It has a name, madarosis, and it has been in the textbooks for a century.

How often it actually happens depends heavily on which study you read. In that same Kashmir series of 460 patients, it was the least common hair finding of all: nine people, 1.96%.¹ Smaller series of newly diagnosed, untreated patients have put it above 20%.¹⁰ Treatment status probably explains most of that spread, since the low figure comes from a clinic where many patients were already on thyroxine.

Either way, the range is wide enough that its absence tells you nothing. Most people with thyroid shedding have perfectly ordinary eyebrows. Mention it if yours are thinning at the outer edge, since it is a useful clue when it is there, but understand that a full set of eyebrows tells your doctor precisely nothing about whether your thyroid is behind the shedding.

Will it grow back? Copy link

Usually, yes. Telogen effluvium is reversible by nature, and once the trigger is removed the follicles restart. The timeline is where expectations break, because it moves slower than almost anyone plans for.

In telogen effluvium generally, the shedding phase runs about three to six months before new hair starts coming through, and it can take many months more for your volume and thickness to return.⁹ That describes the natural course measured from the trigger, and no primary source gives a regrowth clock counted from the day you start thyroid medication. For thyroid shedding the clock effectively restarts once your levels settle, so counting from your first tablet will make recovery look slower than it is.

Hair also grows at roughly a centimetre a month, which sounds fine until you work out what it means for a scalp you are checking daily. Even after regrowth begins, the short new hairs sit below the length of everything else for a while, which is why people often feel nothing is happening when it already is. If you look closely at your parting under a bright light, those short upright hairs standing proud of the rest are the thing to look for. They arrive well before the mirror looks different.

One documented case gives a sense of the pace. A woman with severe uncontrolled hypothyroidism and marked eyelash loss restarted levothyroxine and had visible regrowth by about 12 weeks. Her TSH had fallen a long way by then, though it was still above range.⁸ A single case report, so read it as an illustration and not a schedule.

Why is my hair still falling after starting treatment? Copy link

Most people come back to the doctor around week six with this one, convinced the medicine has failed.

Two things are happening. The first is the lag: the follicles that committed to resting before you started treatment are still going to shed on their own schedule, no matter how right your dose is, so you are watching a decision your body made two months ago finish playing out. Nothing stops that. The second is that your levels may not be where they need to be yet, and dose adjustment takes weeks, with repeat testing in between.

There is also a real possibility that the thyroid was never the only thing going on.

The things that get missed Copy link

Iron is the big one. Low ferritin causes telogen effluvium on its own, it is extremely common in Indian women, and it frequently sits alongside thyroid disease as well as underneath it, which is exactly why treating only the thyroid leaves so many people still shedding six months later. In a study of 100 Indian women, ferritin was significantly lower in those with telogen effluvium than in controls.⁷ A small single-centre study, so read it as support for checking rather than a threshold to treat by. If your thyroid is treated and corrected and the shedding continues, ferritin is the first thing to check. Note that a normal haemoglobin does not rule it out, because your stores can be depleted long before your count drops. A complete blood count gives you part of the picture; ferritin has to be asked for separately.

Beyond that, the usual suspects are a crash diet or rapid weight loss, a delivery in the last year, a bad viral illness or surgery, a new medication, and low vitamin D. That last one is close to a default finding in Indian adults, so it deserves careful interpretation before anyone treats it on reflex.

Antithyroid drugs used for an overactive thyroid deserve their own mention, because the treatment itself can contribute. Hair loss is a described side effect of methimazole and propylthiouracil, and so of carbimazole, which your body converts to methimazole. The evidence for it is thinner than that sentence sounds. The clearest statement of it comes from a 1999 review, which lists the effect among minor ones without quantifying it.⁴ Recent pharmacovigilance work on these drugs surfaces agranulocytosis, liver injury and vasculitis. Alopecia does not show up as a signal. Frankly, if your hair is shedding while you are on one of these, uncontrolled thyroid levels remain the more likely explanation. Talk to your doctor before changing anything.

Which tests are worth asking for? Copy link

One blood test settles the thyroid question, and TSH is the number that does most of the work. Your doctor may add free T4 alongside it, and anti-TPO antibodies if there is reason to suspect autoimmune thyroid disease. If you want to know what each of those measures before you go, our guide to thyroid testing walks through them.

Alongside those, ferritin is the one that most often changes what happens next, and vitamin D is worth having if it has not been checked recently. As you would expect from the list above, the point of testing is not only to confirm the thyroid but to find the other thing sitting underneath it.

What about weight, since both changed together? Copy link

Many people arrive with hair shedding and weight gain at the same time and assume one tablet will fix both. It usually does not work that way, and the weight side has a much weaker response to treatment than the hair side does. Thyroid and weight goes through the measured numbers on that, including why the early weight change on treatment is mostly fluid.

If your weight is the bigger worry and your thyroid turns out to be well controlled, that becomes a separate conversation with separate options. What to expect from Ozempic is a reasonable place to start reading before any of it.

Worth getting looked at Copy link

See someone if the shedding has run longer than about six months, or if you can see scalp through your parting where you could not before. The same goes if you have the cluster that travels with thyroid disease: persistent fatigue, feeling cold when nobody else does, constipation, heavier or irregular periods, unexplained weight change.

Go sooner if you have a bald patch with a defined edge, if your scalp is red, scaly, painful or itchy, or if hair is coming out in clumps. Those point somewhere other than a simple thyroid effluvium, and they are worth a dermatologist looking at directly.