Does hypothyroidism or thyroid medication cause constipation?

Qassi Gaba · 26 October 2026 · 5 min read

Yes, an underactive thyroid is a recognised cause of constipation. Levothyroxine, taken at the right dose, is not generally one, but iron supplements people often take alongside it are, and that gets confused with the thyroid medication itself.

Those are three separate questions wearing one coat, so it's worth pulling them apart.

Why an underactive thyroid slows things down

Thyroid hormone affects how fast food moves through the gut, among a long list of other things it does. The American Thyroid Association lists constipation as one of the common symptoms of hypothyroidism, alongside fatigue, weight gain and feeling cold. The NHS page on an underactive thyroid lists it too. NIDDK's overview of hypothyroidism describes the same pattern: slower metabolism, slower gut.

This is one of the symptoms that tends to arrive gradually, which is partly why it gets missed. Nobody wakes up one day constipated. It creeps in over months alongside the tiredness and the cold hands, and by the time someone mentions it, they have usually stopped noticing it as a symptom at all and started treating it as just how they are now.

If the underlying cause is Hashimoto's, the thyroid's own decline is usually gradual too, which is part of why the constipation shows up slowly rather than all at once. NIDDK's page on Hashimoto's disease covers how that progression typically works.

What levothyroxine's own label actually says

This is where it gets counterintuitive. Levothyroxine is not usually what causes constipation. It's often what is supposed to be treating it.

The labelling for levothyroxine lists gastrointestinal symptoms among the effects of too much thyroid hormone, not too little: diarrhoea, not constipation, when levels run high. Constipation belongs on the other side of the ledger, as a symptom of underactive thyroid function, which is what the drug exists to correct.

So if someone starts levothyroxine and constipation improves over the following weeks, that's usually the expected direction of travel, as thyroid hormone levels come up toward a normal range. If it persists once a dose has settled, that's a different conversation, and one for whoever is managing the prescription rather than something to work out from a drug label. The NHS page on levothyroxine covers common side effects, and constipation is not among the ones typically attributed to the drug at a correct dose.

Liothyronine and combination therapy

Liothyronine is T3, the more active form of thyroid hormone, sometimes prescribed alongside or instead of levothyroxine's T4. The ATA's page on thyroid hormone treatment covers how the two are used and compared. Because T3 acts faster and is cleared faster than T4, the relationship between dose and symptom differs in timing, but the underlying logic is the same as above: thyroid hormone that is too low tends to slow the gut, not the reverse. Whether a particular combination or dose is contributing to someone's symptoms is a question for the prescriber, not something a drug name alone answers.

The ferritin confusion

Ferritin is not a supplement. It's a measurement, the amount of iron your body has stored, and it's a test some people with Hashimoto's end up having checked because iron deficiency is common and can produce some of the same tiredness that hypothyroidism does. What people usually mean when they ask whether "ferritin" causes constipation is whether an iron supplement, taken because ferritin came back low, does.

It does, for a lot of people. Iron supplements are well known to slow the gut, and this is a separate mechanism from anything happening with thyroid hormone. The NIH Office of Dietary Supplements' iron fact sheet covers what the supplement does and how ferritin is used to assess someone's iron stores.

There's also a practical overlap worth knowing about, separate from the constipation question. Iron can reduce how much levothyroxine the body absorbs if the two are taken too close together, which is why the levothyroxine labelling notes spacing them apart. So somebody taking both is dealing with two different substances, on two different schedules, each capable of affecting the gut for its own reason, and it's easy to end up blaming the wrong one.

Sorting out which one it is

With three plausible causes sitting in the same week, iron, the medication dose, and the underlying thyroid level itself, there's no reliable way to work out which one from the symptom alone. It usually takes someone looking at the labs alongside the timeline: when the constipation started, when a dose changed, when a supplement started, and what TSH and Free T4 were doing across that same stretch.

The British Thyroid Foundation has material aimed at patients living with this day to day, including the kind of symptom tracking that makes a pattern like this visible rather than just felt.

Where this touches Haloscan

This is close to the problem we built the app to help with. If someone logs a symptom like constipation alongside their supplements and their lab results over time, it becomes possible to see that it started the week an iron supplement did, or that it's been present since before any medication at all and tracks instead with TSH. The two-page report pulls that timeline together for an appointment, rather than leaving someone to reconstruct it from memory in the waiting room.