Does hypothyroidism cause hair loss, and can the medication cause it too?

Qassi Gaba · 19 October 2026 · 4 min read

Yes, both things are real. An underactive thyroid can cause hair loss, and starting thyroid medication can cause a separate round of hair loss a few months later, one that usually settles on its own. They have different causes and different timelines, which is why people get confused about which one they are dealing with.

Hair loss from the thyroid itself

Hair follicles need thyroid hormone to go through their normal cycle. When levels are low, more hairs than usual sit in the resting phase at once instead of cycling through growth, and they shed together rather than gradually. The result is hair that feels thinner all over the scalp, not in patches, and comes out more than usual in the shower or on a brush.

Hypothyroidism is the general name for an underactive thyroid, and hair thinning is listed among its recognised features by both the American Thyroid Association and the NHS. It tends to show up alongside other things: fatigue, weight change, feeling cold, dry skin. Hair loss on its own, without any of those, is less obviously a thyroid story.

Hashimoto's thyroiditis is the most common cause of an underactive thyroid in places with enough dietary iodine, and it causes hair loss through the same route: the gland produces less hormone over time as the immune system affects it. The hair loss itself isn't a separate process from the hypothyroidism it produces.

The thyroid is not the only reason hair sheds this way. Low iron, pregnancy and the months after it, some medications, and stress on the body from illness or surgery can all produce a similar diffuse shedding. A doctor sorting out which one applies will usually want to see more than a TSH.

Hair loss from starting the medication

This is the part that surprises people and makes them suspect the pills rather than the disease. A few months into treatment with levothyroxine, some people notice more shedding, not less. It can feel like the opposite of what they expected, and it's common enough that it's listed in the drug's own prescribing information as a reported effect.

One explanation is that correcting a hormone level, in either direction, can push a batch of follicles into the resting phase at once, the same mechanism as the hypothyroidism itself, just triggered by the change rather than by the deficiency. It's usually temporary and tends to resolve within a few months as the hair cycle settles back to normal.

Liothyronine, the other thyroid hormone sometimes prescribed alongside or instead of levothyroxine, has the same listing in its own labelling. Synthroid is a specific brand of levothyroxine, and its FDA-approved labelling covers the same drug, so the same effect applies.

Telling the two apart from the inside is hard, because both can produce the same diffuse, whole-scalp shedding. What differs is the timing: hair loss that predates treatment points toward the underlying thyroid condition, hair loss that starts a couple of months after a dose change points toward the adjustment itself. Neither is something to work out alone. It's the kind of pattern a doctor sorts by looking at when the shedding started relative to when the dose changed, alongside the lab results from both points in time.

What a lab result does and does not tell you

TSH and free T4 are the usual tests used to assess thyroid function, and thyroid antibody tests are used to look for Hashimoto's specifically. None of them measure hair directly, and a result inside the reference range does not rule out that the thyroid is contributing to what someone is seeing in the shower drain. The reverse is also true: a result outside the range does not confirm that the thyroid is the explanation for it, since several other common causes produce an identical pattern of shedding.

What the numbers are good for is tracking change over time, particularly across a dose adjustment, which is exactly the period when this kind of hair loss tends to show up.

Where this fits with Haloscan

We built Haloscan to make that tracking easier, because the connection between a dose change and something that shows up three months later is easy to lose without a record of when each happened. The app logs results and medication changes on the same timeline, and symptom logging covers things like hair loss, so a pattern across a dose adjustment is something you can actually show your doctor rather than describe from memory.