If your thyroid symptoms get louder when your digestion is off, you are not imagining a connection. Researchers refer to a gut-thyroid axis, and it is one of the reasons functional medicine looks at digestion when someone has thyroid issues.
What the research suggests
Several studies have found differences in the gut bacteria of people with autoimmune thyroid disease compared with people without it. Celiac disease, which damages the lining of the small intestine, is more common in people with Hashimoto's than in the general population. The gut also plays a role in absorbing the nutrients the thyroid relies on, including iron, selenium, zinc, and the medication levothyroxine itself.
It is worth being straight about what is not known. These are associations, and research has not shown that fixing the gut will cure a thyroid condition. What it does suggest is that a healthy gut gives your body better conditions to work with.
How gut problems can make symptoms worse
- Poor absorption: conditions like celiac disease, low stomach acid, or inflammation can reduce absorption of iron, B12, and thyroid medication, so your labs and symptoms do not improve the way they should. - Immune activation: when the gut barrier is irritated, the immune system can be on higher alert, which matters in autoimmune conditions. - Stress and slow digestion: constipation, bloating, and reflux are common with an underactive thyroid and add to the feeling of being unwell.
Signs your gut may be involved
Persistent bloating, constipation or diarrhea, reflux, food reactions that are getting worse, low iron that does not improve, or thyroid medication that never seems to work well.
Gut-first steps that are safe to begin
- Ask about a celiac blood test before you remove gluten, because you need to be eating it for the test to work. - Eat a variety of plants and aim for 25 grams of fiber, adding it slowly. - Include fermented foods if you tolerate them. - Eat meals slowly, sitting down, and chew well. - Cut back on alcohol and ultra-processed food. - Walk after meals and manage stress. - Ask your clinician about checking iron, ferritin, B12, and vitamin D.
Medication tip
If you take levothyroxine, take it consistently, on an empty stomach, apart from coffee and from calcium or iron, as your clinician advises. Changes in digestion can change how much you absorb.
A note on stool tests and supplements
Commercial gut tests are not standardized, and supplements can be expensive and unnecessary. Start with food, sleep, and stress, and talk with a clinician or dietitian who knows thyroid care before adding anything.
When to see a doctor
See a clinician for blood in the stool, unintended weight loss, persistent diarrhea, severe pain, or anemia.
A note from Tia: I healed my own thyroid by changing how I eat, sleep, manage stress, and support my gut, and five years later my markers are stable and I am medication-free. That is my personal story, not a promise or a prescription. Please work with your clinician, keep taking any prescribed medication unless they tell you otherwise, and use this as a way to ask better questions.
The bottom line
Gut health is not the whole story with the thyroid, but it is a part you can influence. For a practical next step, try the Purposeful Gut Reset.