Hypothyroidism can cause stomach pain, and it does so more often than most people realize. Constipation is the digestive symptom most commonly linked to an underactive thyroid, but abdominal pain, bloating, nausea, and vomiting all appear in hypothyroid patients. The connection runs through several pathways: your digestive tract slows down, bacteria can overgrow in the small intestine, and related autoimmune conditions may cause their own gut symptoms. When standard GI workups come back normal, thyroid function is one of the things worth checking.
How an Underactive Thyroid Slows Your Gut
Thyroid hormones influence the speed at which food moves through your entire digestive system. When those hormones drop, everything slows. In one study comparing hypothyroid patients to healthy controls, food took an average of 49 minutes to empty from the stomach in hypothyroid patients versus 30 minutes in the control group. Esophageal transit time was roughly double as well, at about 53 seconds compared to 24 seconds in healthy subjects.
The slowdown isn’t limited to the stomach. Reduced movement, or peristalsis, occurs throughout the esophagus, small intestine, and colon. The leading explanation involves a substance called hyaluronic acid, which accumulates in the smooth muscle and surrounding tissues of the GI tract when thyroid hormones are low. This buildup causes swelling in the intestinal wall that physically impairs the muscle contractions needed to push food along. Other contributing factors include changes in nerve signaling to the gut and reduced sensitivity of certain receptors that help coordinate digestion.
This sluggish movement is what causes the constipation so strongly associated with hypothyroidism, but it also traps gas, creates pressure, and leads to the kind of vague abdominal discomfort and bloating that can easily be mistaken for irritable bowel syndrome.
Bacterial Overgrowth in the Small Intestine
When the gut moves slowly, bacteria that normally live in the large intestine can migrate upward and multiply in the small intestine. This condition, called small intestinal bacterial overgrowth (SIBO), may be present in more than half of people with hypothyroidism. One study found that 54% of hypothyroid patients tested positive for SIBO on a breath test, compared to just 5% of controls.
SIBO produces its own set of symptoms: bloating, flatulence, abdominal discomfort, and sometimes diarrhea. This is one reason some hypothyroid patients experience diarrhea rather than constipation, which can be confusing if you expect an underactive thyroid to cause only constipation. The excess bacteria ferment food in the small intestine, generating gas that distends the bowel and creates cramping pain. These symptoms often improve with antibiotic treatment targeting the overgrowth, but they tend to recur if the underlying slow motility isn’t addressed through thyroid hormone replacement.
Reduced Stomach Acid Production
Hypothyroidism can also reduce the amount of acid your stomach produces. Lower acid levels impair the initial breakdown of food, particularly proteins, which can lead to a feeling of heaviness, nausea, or upper abdominal discomfort after eating. This reduced acid output may be partly driven by autoimmune gastritis, a condition where the immune system attacks the acid-producing cells of the stomach lining. Since the most common cause of hypothyroidism (Hashimoto’s thyroiditis) is itself autoimmune, having one autoimmune condition raises the likelihood of developing another.
Autoimmune Conditions That Overlap
People with autoimmune thyroid disease have a higher-than-average chance of also having celiac disease, an autoimmune reaction to gluten that damages the small intestine. Estimates suggest 2 to 5% of people with autoimmune thyroid conditions also have celiac disease, though some studies have found rates closer to 9%. The two conditions share genetic risk factors.
What makes this tricky is that celiac disease can be completely silent or present with nonspecific symptoms like abdominal pain, iron deficiency, or fatigue, symptoms that easily get attributed to the thyroid problem itself. In one study of autoimmune thyroid patients who tested positive for celiac markers, most had no classic digestive complaints like diarrhea. If you have Hashimoto’s and persistent stomach pain that doesn’t improve with thyroid treatment, celiac screening is worth pursuing.
Your Thyroid Medication Could Be Contributing
Here’s an overlooked possibility: the very medication used to treat hypothyroidism can sometimes cause stomach problems of its own. Many levothyroxine tablets contain lactose as a filler ingredient. For people with lactose intolerance, this can trigger flatulence, abdominal pain, bloating, and diarrhea. The symptoms show up after each dose and can be mistaken for ongoing thyroid-related gut issues.
If you suspect this is happening, lactose-free liquid formulations of levothyroxine exist and have resolved GI symptoms in patients who made the switch. It’s also worth noting that taking too much thyroid medication, pushing your body into a mildly hyperthyroid state, can cause diarrhea and abdominal cramping. Periodic blood work helps ensure your dose stays in the right range.
Severe Cases: Myxedema and Megacolon
In rare, severely untreated hypothyroidism, the colon can become massively dilated, a condition called myxedema megacolon. This typically presents as significant abdominal distention, worsening constipation, and flatulence. In extreme cases, the stretched colon wall can lose blood supply, leading to tissue damage and intense abdominal pain. One documented case involved a patient with over a year of bloating, abdominal pain, and intermittent diarrhea before the underlying severe hypothyroidism was identified. Symptoms resolved with thyroid hormone replacement. This complication is uncommon, but it underscores what can happen when hypothyroidism goes untreated for a long time.
Why It Often Gets Missed
Stomach pain from hypothyroidism is frequently attributed to other causes first. Clinicians naturally consider peptic ulcers, gallbladder disease, pancreatitis, and inflammatory bowel disease before thinking about the thyroid. In published case reports, patients with nausea, vomiting, and abdominal pain have gone through extensive GI workups only to discover that primary hypothyroidism was the cause all along. The pattern that raises suspicion is refractory GI symptoms, meaning symptoms that persist despite standard treatment and don’t match a clear GI diagnosis.
The good news is that most thyroid-related digestive problems improve with proper thyroid hormone replacement. Studies have shown that reduced motility in the stomach, small intestine, and colon reverses once thyroid levels normalize. The timeline varies, but many people notice digestive improvements within weeks to a few months of reaching stable thyroid hormone levels. For complications like SIBO, additional targeted treatment may be needed alongside thyroid management to fully resolve symptoms.