Can hypothyroidism or thyroid medication cause weight gain?

Qassi Gaba · 13 October 2026 · 5 min read

Short answer: an underactive thyroid can cause some weight gain, but it is usually modest, a few kilos, much of it water rather than fat. The medications used to treat it, levothyroxine, liothyronine, Synthroid, are not generally linked to weight gain. If anything, correcting an underactive thyroid tends to be followed by a small amount of weight loss, not gain.

That is the whole answer. Everything below is about why, and about the questions hiding inside this one.

What the thyroid actually does to your weight

The thyroid makes hormones that set the pace of your metabolism, roughly how fast your body burns energy at rest. When output drops, which is what hypothyroidism means, that resting burn slows down. Less of what you eat gets used, and some of it is retained as water and salt rather than fat, because thyroid hormone also affects how the kidneys handle fluid.

The NHS and the American Thyroid Association both list weight gain among the recognised symptoms of an underactive thyroid, alongside tiredness, feeling cold, and constipation. Neither describes it as dramatic. The ATA has a page specifically on thyroid and weight which puts a figure on it: "about 5-10 pounds of weight gain may be related to the thyroid, depending on the severity of the hypothyroidism", and "most of the weight gained is actually due to retaining salt and water".

Five to ten pounds, mostly water, is the number worth holding on to. A thyroid problem can be part of why the scale moved. On the ATA's own figures it is rarely all of it.

Does Hashimoto's specifically make this worse

Hashimoto's thyroiditis is the autoimmune condition that is the most common cause of hypothyroidism in places where iodine intake is adequate. The ATA's page on Hashimoto's describes the immune system gradually damaging the gland, which is why hormone output falls over time rather than all at once.

Because of that gradual course, weight change from Hashimoto's tends to track however underactive the thyroid has become at a given point, rather than being a separate effect of the autoimmune process itself. Somebody with Hashimoto's and normal hormone levels, caught early through antibody testing, is not described in the guidance as having weight gain from the condition. Somebody whose levels have drifted into hypothyroid territory is.

Does levothyroxine cause weight gain

Levothyroxine is the standard treatment for hypothyroidism, a synthetic version of the T4 hormone the thyroid normally makes. Its purpose is to replace what the gland is no longer producing enough of, which is the opposite of what would cause weight gain.

NICE's clinical knowledge summary for hypothyroidism covers treatment with levothyroxine and does not list weight gain as an expected effect of the drug itself. The common pattern described in the literature is the reverse: people often lose a small amount of weight, largely fluid, once treatment brings hormone levels back into range. The NHS's page on levothyroxine lists its common side effects, and weight gain is not among them. Synthroid is a brand name for the same drug, so the same applies to it.

Where people do report weight change around starting levothyroxine, it is worth separating two different things: the thyroid problem that existed before treatment, and the treatment itself. It is easy to blame the pill for what was really caused by months or years of untreated hypothyroidism beforehand.

What about liothyronine

Liothyronine is a synthetic form of T3, the more active thyroid hormone, sometimes used alongside or instead of levothyroxine. The ATA's page on thyroid hormone treatment discusses combination therapy with T3 and T4 and notes that most studies have not shown it to outperform levothyroxine alone for the symptoms people hope it will help, weight included. It does not describe liothyronine as a cause of weight gain.

Reverse T3 and weight

Reverse T3 is a mirror-image molecule the body makes from T4, inactive rather than hormone-like. It gets raised online as an explanation for why weight will not shift despite a normal TSH. The ATA's explanation of thyroid function tests does not include reverse T3 among the tests it recommends, because its level does not reliably track how the body is using thyroid hormone the way TSH and Free T4 do. No guideline body currently recommends testing it for weight concerns.

Do thyroid nodules cause weight gain

A thyroid nodule is a lump in the gland. Most are found incidentally and most do not change how much hormone the gland produces. Weight is only affected if a nodule happens to be one of the minority that pushes hormone output up or down, which shows up as hyperthyroidism or hypothyroidism on a blood test, the same two conditions already described above. A nodule that leaves hormone levels normal is not expected to move the scale on its own.

What this does and doesn't tell you

None of this tells you whether your own weight change is from your thyroid, from your medication, or from something else entirely. That is a question for the person treating you, and it usually needs a blood test alongside the conversation, not a guess from symptoms alone.

Where this touches Haloscan

Haloscan logs weight with the rest of your symptom entries, on the same timeline as your TSH and Free T4 results, because the two get discussed in isolation from each other at appointments. Whether a few kilos arrived before a dose change or after it is the kind of question a date answers and memory does not.