Can an underactive thyroid cause headaches?

Qassi Gaba · 22 October 2026 · 5 min read

Short answer: not usually, on its own. Headache isn't on the standard symptom lists for an underactive thyroid published by the American Thyroid Association, NHS or NIDDK. But people with a thyroid condition report headaches often enough, and there are two places in this picture where headache genuinely belongs. Neither of them is "the thyroid hormone itself caused it."

What the guidance actually lists

The symptoms that keep showing up across the ATA, NHS and NIDDK pages are fatigue, weight gain, feeling cold, dry skin, constipation, low mood, and muscle aches. The ATA's page on Hashimoto's thyroiditis, the autoimmune condition behind most hypothyroidism, describes the same cluster. Headache isn't on any of these lists as a core feature, but that doesn't mean it never happens to someone with the condition. It's a common thing in the general population regardless of thyroid status, so its absence from these lists mostly reflects that it isn't specific enough to hypothyroidism to count as a sign of it.

Where headache does belong: the pituitary

TSH, the number your doctor is usually watching, isn't made by the thyroid. It's made by the pituitary gland, which sits behind the eyes and tells the thyroid how much hormone to produce. The ATA's explanation of thyroid function tests covers this loop: low thyroid hormone normally pushes TSH up, because the pituitary is trying to get more hormone out of the thyroid.

Occasionally the fault is in the pituitary itself rather than the thyroid. NIDDK's page on hypothyroidism describes this as a less common cause, usually from a growth on the pituitary. A growth there can press on surrounding structures and cause headache directly, sometimes along with vision changes, and it can also disrupt the signal to the thyroid and produce hypothyroidism as a side effect of the same problem. In that case the headache and the low thyroid hormone share a cause. The low thyroid hormone isn't what's giving you the headache.

This is a small fraction of hypothyroidism overall. Most of it is Hashimoto's, where the thyroid itself is the site of the problem and the pituitary is working normally, just pushing out more TSH than usual to compensate.

Levothyroxine's own label

Headache does appear on the safety information for levothyroxine, the standard treatment for hypothyroidism. The drug's current US labelling lists headache among the adverse reactions reported, alongside things like insomnia and tremor, most of which read as symptoms of getting more thyroid hormone than the body is currently used to. That's distinct from hypothyroidism causing a headache. It's closer to the opposite: an adjustment period, often around a dose change, in which the body is recalibrating to a different level of hormone than before.

This also means headache can turn up after things that are going right. A dose increase that is correcting a genuinely low level can still coincide with a week or two of feeling off before things settle, and a headache during that window isn't automatically a sign that anything is wrong.

The overactive comparison

It's worth knowing that headache sits closer to the hyperthyroid side of things than the hypothyroid side. NHS's page on an overactive thyroid lists symptoms like a fast or irregular heartbeat, nervousness, and weight loss despite eating normally, a cluster that is more commonly associated with headache in practice than an underactive thyroid is. This matters because over-replacement, taking more levothyroxine than the body needs, pushes someone's chemistry in that direction. A headache that starts after a dose increase is one of the things worth mentioning at the next appointment, not because it proves anything, but because the timing is a detail your doctor can't see unless you report it.

What this means if you have one

A headache by itself doesn't tell you much about your thyroid. What's more useful is the pattern around it: whether it started near a dose change, whether it comes with other new symptoms, and whether it's a new kind of headache for you or one you've always had. None of that is something this article, or any article, can read for you. It's a question for whoever is managing your treatment, and it's one they can only answer well if they have the timeline in front of them.

Where this touches Haloscan

This is part of why Haloscan lets you log symptoms alongside doses and lab draws rather than as a separate note somewhere. A headache that shows up the week of a dose increase looks different on a timeline than one that has been there for years regardless of what your levothyroxine dose is doing. The app doesn't tell you which one yours is. It just makes sure the pattern is visible when you sit down with your doctor, instead of being something you're trying to remember off the top of your head in the appointment itself.