Hypothyroidism and Constipation: The Gut-Thyroid Link

Why does an underactive thyroid slow your digestion?

Thyroid hormones act on the autonomic nervous system and the smooth muscle of the digestive tract. When thyroid hormone is low, peristalsis — the coordinated muscle contractions that push food through the intestines — slows down. The result is that food and stool move more slowly through the colon, allowing more water to be absorbed, producing hard, dry, difficult-to-pass stool 3Ref 3Xu GM, Hu MX, Li SY, Ran X, Zhang H, Ding XF (2024).Thyroid disorders and gastrointestinal dysmotility: an old association.Hypothyroidism slows colonic transit via reduced smooth-muscle activity; glycosaminoglycan accumulation delays intestinal transit; ~54% of hypothyroid patients show bacterial overgrowth; thyroid hormone replacement improves GI motility problems.

A 2024 review of thyroid disorders and gastrointestinal dysmotility found that glycosaminoglycan accumulation in smooth muscles (a hallmark of hypothyroidism) delays intestinal transit, and that nearly 54% of hypothyroid patients show evidence of bacterial overgrowth — which contributes to bloating and discomfort 3Ref 3Xu GM, Hu MX, Li SY, Ran X, Zhang H, Ding XF (2024).Thyroid disorders and gastrointestinal dysmotility: an old association.Hypothyroidism slows colonic transit via reduced smooth-muscle activity; glycosaminoglycan accumulation delays intestinal transit; ~54% of hypothyroid patients show bacterial overgrowth; thyroid hormone replacement improves GI motility problems. In severe or untreated cases, slowing can progress to megacolon or paralytic ileus, though these are uncommon.

Constipation is one of the most common gastrointestinal symptoms reported by people with hypothyroidism, along with bloating and abdominal fullness. It often appears alongside other hallmark symptoms like fatigue, cold intolerance, and weight gain — but in some people, especially in milder hypothyroidism, digestive slowness may be among the first or most noticeable complaints. Hypothyroidism affects an estimated 9–12% of U.S. adults, with prevalence higher in women and rising with age 4Ref 4Wyne KL, Nair L, Schneiderman CP, Pinsky B, Antunez Flores O, Guo D, Barger B, Tessnow AH (2022).Hypothyroidism Prevalence in the United States: A Retrospective Study Combining National Health and Nutrition Examination Survey and Claims Data, 2009–2019.Hypothyroidism prevalence in the U.S. was ~9.5–11.7% from 2009–2019 and has been rising; higher prevalence in women and with increasing age; >78% of patients receive levothyroxine monotherapy.

Will treating the thyroid fix the constipation?

Often, yes — at least partially. When levothyroxine brings TSH into the normal range, gut motility typically improves 1Ref 1Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Levothyroxine as standard treatment, TSH normalization, and the expectation that symptoms including gastrointestinal symptoms improve with treatment. Thyroid hormone replacement therapy has a beneficial effect on thyroid-disease-related GI motility problems 3Ref 3Xu GM, Hu MX, Li SY, Ran X, Zhang H, Ding XF (2024).Thyroid disorders and gastrointestinal dysmotility: an old association.Hypothyroidism slows colonic transit via reduced smooth-muscle activity; glycosaminoglycan accumulation delays intestinal transit; ~54% of hypothyroid patients show bacterial overgrowth; thyroid hormone replacement improves GI motility problems. Some people notice their bowel pattern normalizing within weeks of starting or adjusting thyroid treatment. However, the improvement is not always complete — especially if constipation was already present before the thyroid condition, if the dose is not yet optimized, or if other contributing factors (low fiber diet, dehydration, inactivity) remain in place.

What else can help constipation when you have hypothyroidism?

Dietary and lifestyle measures that support bowel regularity are the same whether or not thyroid disease is present:

Fiber: Aim for adequate daily fiber from whole foods — vegetables, fruits, legumes, whole grains. Both soluble fiber (oats, beans, flaxseed) and insoluble fiber (wheat bran, vegetables) contribute to regularity. Most adults fall well short of recommended daily intake.

Hydration: The colon reabsorbs water from stool; adequate fluid intake prevents stool from becoming too dry. Six to eight glasses of water daily is a reasonable starting point, though individual needs vary.

Physical activity: Movement stimulates bowel motility. Even regular walking has a meaningful effect on transit time.

Consistent toilet habits: Responding to the urge when it occurs (rather than delaying) and allowing unhurried time in the morning can help.

Osmotic laxatives: If lifestyle measures are insufficient, over-the-counter osmotic agents (polyethylene glycol, magnesium citrate) are first-line pharmacological options for chronic constipation per gastroenterology guidelines 2Ref 2Chang L, Chey WD, Imdad A, et al. (2023).American Gastroenterological Association-American College of Gastroenterology Clinical Practice Guideline: Pharmacological Management of Chronic Idiopathic Constipation.Osmotic laxatives (polyethylene glycol, magnesium salts) as first-line pharmacological options for chronic constipation per AGA-ACG guidelines. These draw water into the colon to soften stool and do not cause dependency. Stimulant laxatives are for short-term use only; discuss regular use with a clinician.

When should digestive symptoms prompt a clinician visit?

If you have hypothyroidism and constipation that does not improve with optimized thyroid treatment and lifestyle changes, a conversation with your Gale clinician is warranted. Other reasons to seek evaluation include:

• Constipation that is new and unexplained without a known thyroid diagnosis
• Blood in the stool or unexplained weight loss alongside bowel changes
• Constipation alternating with diarrhea (which may suggest irritable bowel syndrome or another diagnosis)
• Severe abdominal pain or inability to pass stool at all

The key is not to assume that all digestive symptoms are thyroid-related — a clinician review of your TSH alongside a bowel symptom history gives the clearest picture.