Can thyroid medication cause diarrhea?

Qassi Gaba · 29 October 2026 · 5 min read

Yes. Levothyroxine, liothyronine and brand versions like Synthroid can all be linked to diarrhea, though the reason differs depending on what is actually going on. The most common one is simply too much hormone for that person's dose.

That is not the only explanation, and it is not something to work out from a search result. But it is worth knowing what the possibilities are, because "my medication is upsetting my stomach" and "my dose needs a look" are two different conversations with a doctor.

Too much thyroid hormone looks like this

Levothyroxine and liothyronine both replace a hormone your thyroid is not making enough of. If the replacement amount runs ahead of what the body needs, the result is a set of symptoms that overlaps heavily with an overactive thyroid: a faster heartbeat, weight loss, feeling hot, anxiety, and a change in bowel habits that more often means looser or more frequent stools. The ATA's page on hyperthyroidism lists "frequent bowel movements or diarrhea" among the symptoms of excess thyroid hormone, and NIDDK's names frequent bowel movements the same way, whatever the source of that excess.

Liothyronine is the more potent and faster-acting of the two hormones. The ATA's page on thyroid hormone treatment describes how T3 preparations produce a sharper rise and fall in blood levels than T4 alone, which is part of why they are prescribed more cautiously and in smaller amounts. A sharper peak means a narrower margin before symptoms of excess appear.

Too little can also show up as a change in digestion

This is the part that surprises people. Untreated or undertreated hypothyroidism is more classically linked to constipation, not diarrhea. The NHS page on an underactive thyroid lists constipation among the common symptoms of too little thyroid hormone. So new diarrhea is less likely to mean undertreatment and more likely to point toward the opposite problem, or toward something unrelated to the dose at all. Either way, it is a pattern a doctor reads against a lab result, not a guess made from the symptom alone.

The label itself

Drug labeling for levothyroxine, searchable through DailyMed's levothyroxine listings, lists gastrointestinal symptoms including diarrhea among the adverse reactions reported, generally in the context of excessive dosage producing symptoms of thyrotoxicosis. Synthroid is one of several brand products sold under that same active ingredient, so its labeling sits inside that same search. Different manufacturers' versions of levothyroxine are approved separately by the FDA, and the FDA's drug approval database shows that these products are not identical in every respect: the active hormone is the same, but the inactive ingredients, the fillers, dyes and binders that hold the tablet together, can differ between manufacturers and between a brand and its generic. Some people report a reaction after switching brands, though this is harder to study than the dose itself, and the FDA's own stance is that approved generics are required to be bioequivalent.

Taking it on an empty stomach

Levothyroxine absorption is sensitive to food, coffee and other medications taken around the same time. The NHS guidance on levothyroxine describes taking it on an empty stomach, usually first thing in the morning, with a gap before eating. Taken a different way each day, absorption can vary from one dose to the next, which is a separate mechanism from the dose itself being wrong, but can still produce symptoms that look dose-related when they are really a timing and consistency issue.

An unrelated condition riding along

Hashimoto's is autoimmune, and autoimmune conditions have a habit of traveling in clusters. The NIDDK's page on Hashimoto's disease puts the link this way: you are more likely to develop Hashimoto's if you already have another autoimmune disorder, and celiac disease is the first one it names. Celiac disease causes diarrhea through an entirely separate mechanism, damage to the gut lining from gluten, and it has nothing to do with the thyroid dose at all. This is one reason a new digestive symptom in someone with Hashimoto's is worth raising rather than assuming it belongs to the medication by default.

What this actually comes down to

New diarrhea after starting or changing a thyroid medication has several possible explanations behind it: too much hormone, a reaction to a specific formulation, inconsistent timing around food, or something unrelated that happens to share a patient. These point in different directions, and some of them are checked with a blood test, not a guess. None of them are something to sort out by elimination at home.

Where this touches Haloscan

Haloscan logs the medication you are taking alongside the symptoms you record, so if a GI symptom starts the same week as a dose change or a brand switch, that sits next to it on the report rather than buried in two separate places you have to remember to mention. It does not tell you what caused it. It just makes sure the timeline is there when you bring it up.