Your thyroid is a small, butterfly-shaped gland in your neck, but it helps set the pace for much of your body, including your gut. When it makes too little hormone, digestion tends to slow down. When it makes too much, things tend to speed up (Ebert, 2010).
Hypothyroidism is common (Chaker et al., 2017), and thyroid symptoms can overlap with what many women chalk up to midlife: tiredness, weight changes and constipation. Here's how the two main thyroid problems compare when it comes to digestion.
The big picture
A review of thyroid disease and the gut concluded that digestive symptoms are mostly explained by four things. They are slower gut movement in hypothyroidism, faster gut movement in hyperthyroidism, autoimmune inflammation of the stomach lining, and pressure on the esophagus from a thyroid problem. The same review notes that digestive symptoms usually resolve once the thyroid condition is treated (Ebert, 2010).
Underactive thyroid (hypothyroidism)
Common symptoms overall: In adults, the most common symptoms include fatigue, lethargy, feeling cold, weight gain, constipation, voice changes and dry skin (Chaker et al., 2017).
What can happen in the gut (Ebert, 2010):
Constipation, from slower movement of the intestines
Indigestion, nausea or vomiting, which may be related to slower stomach emptying
Heartburn or trouble swallowing, linked in some people with Hashimoto disease to changes in how the esophagus moves
Bloating, gas and abdominal discomfort in people with bacterial overgrowth, which improved with antibiotics
That last point was explored in a small study of 50 people with a history of overt hypothyroidism due to autoimmune thyroiditis. Fifty-four percent had a positive glucose breath test for bacterial overgrowth, compared with 5% of 40 controls. Bloating, gas and abdominal discomfort were more common in those who tested positive, and these symptoms improved after antibiotic treatment (Lauritano et al., 2007).
Overactive thyroid (hyperthyroidism)
What can happen in the gut (Ebert, 2010):
Diarrhea or more frequent stools, because food moves faster from the mouth to the large intestine
Fatty stools in some people, related to eating much more than usual and to stimulation of the nervous system
Normal stomach emptying but lower stomach acid, partly related to autoimmune gastritis
Graves disease accounts for 60% to 80% of cases of thyrotoxicosis, the state of having too much thyroid hormone (Ebert, 2010). If you notice loose stools along with a racing heart, shakiness or feeling unusually hot, mention all of it to your doctor.
Side by side
Bowel habits: constipation with an underactive thyroid; diarrhea or more frequent stools with an overactive one.
Speed: slower gut movement with hypothyroidism; faster transit with hyperthyroidism.
Stomach: slower emptying may bring nausea or indigestion with hypothyroidism; emptying is usually normal with hyperthyroidism.
Bloating: linked mainly with hypothyroidism, sometimes alongside bacterial overgrowth.
After treatment: digestive symptoms usually improve in both (Ebert, 2010).
The celiac connection
Autoimmune thyroid disease and celiac disease show up together more often than chance would predict. A meta-analysis of 6,024 people with autoimmune thyroid disease found biopsy-confirmed celiac disease in 1.6%, about 1 in 62. In adults, the figure was 2.7% (Roy et al., 2016).
The authors argued that people with autoimmune thyroid disease should be screened for celiac disease. If you have Hashimoto or Graves disease plus ongoing bloating, diarrhea or iron deficiency, it's reasonable to ask whether a celiac blood test makes sense.
If you take levothyroxine, timing matters
Thyroid hormone replacement is absorbed in the gut, so what you take it with can make a difference. In a small study, swallowing levothyroxine with coffee reduced its absorption compared with water, lowering one key absorption measure by 27% to 36%. Taking it with water and waiting 60 minutes before having coffee gave results similar to water alone (Benvenga et al., 2008).
If your dose keeps needing adjustment, or you've recently started a new supplement or changed your routine, mention it. Your doctor may want to check your levels or review how and when you take your medication.
When to ask about a thyroid test
New constipation along with fatigue, feeling cold, weight gain or dry skin
New diarrhea along with a racing heart, shakiness or feeling unusually hot
A family history of thyroid disease, or another autoimmune condition
Digestive symptoms that don't respond to the usual fiber, fluid and lifestyle steps
As always, red flags such as blood in your stool, unexplained weight loss, anemia or trouble swallowing need prompt attention, whether or not your thyroid is involved.
Sources
Ebert EC. The thyroid and the gut. J Clin Gastroenterol. 2010;44(6):402-6. PMID 20351569
Lauritano EC, Bilotta AL, Gabrielli M, et al. Association between hypothyroidism and small intestinal bacterial overgrowth. J Clin Endocrinol Metab. 2007;92(11):4180-4. PMID 17698907
Roy A, Laszkowska M, Sundström J, et al. Prevalence of celiac disease in patients with autoimmune thyroid disease: a meta-analysis. Thyroid. 2016;26(7):880-90. PMID 27256300
Benvenga S, Bartolone L, Pappalardo MA, et al. Altered intestinal absorption of L-thyroxine caused by coffee. Thyroid. 2008;18(3):293-301. PMID 18341376
This article is for general information and isn't medical advice. Talk to your healthcare provider about thyroid testing, and don't change how you take thyroid medication without their guidance.
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