The Link Between Thyroid, Gut and Mood

What the thyroid does

The thyroid is a gland at the front of the neck. It makes two hormones, T4 and T3, which set the speed of metabolism in nearly every tissue. The pituitary gland controls it with a signal called TSH. When thyroid hormone is low, TSH rises. When thyroid hormone is high, TSH falls. That is why TSH is the first test.

About 1 in 20 Americans has an underactive thyroid, and women are affected several times more often than men. The American Thyroid Association estimates that up to 60% of people with thyroid disease do not know they have it. In the United States the most common cause of an underactive thyroid is Hashimoto's disease, in which the immune system attacks the gland.

How the thyroid affects the gut

Underactive thyroid (hypothyroidism). The muscles of the digestive tract slow down. The result is constipation, bloating and a feeling of fullness. Slow movement through the small intestine also allows bacteria to build up there. In one study, bacterial overgrowth was found in about half of people with a history of hypothyroidism, compared with about 1 in 20 of those without.

Overactive thyroid (hyperthyroidism). Everything speeds up. Bowel movements become frequent and loose, appetite rises, and weight falls.

These gut symptoms usually improve once thyroid levels are brought back to normal.

How the thyroid affects mood and thinking

Underactive: low mood, fatigue, slowed thinking, poor memory and concentration, and loss of interest. It can look exactly like depression, which is why guidelines advise checking thyroid function in anyone being evaluated for depression.

Overactive: anxiety, irritability, restlessness, trouble sleeping, a racing heart and tremor. It can be mistaken for an anxiety disorder or panic attacks.

When clear thyroid disease is treated, mood and thinking usually improve. One caution: in people with only a mildly raised TSH and normal thyroid hormone, called subclinical hypothyroidism, a large trial in older adults found that treatment did not improve tiredness or other symptoms. A borderline result does not always explain how someone feels.

How gut health affects the thyroid

The gut microbiome plays an important part in thyroid health. A balanced microbiome supports the immune system and keeps inflammation down. An imbalance, called dysbiosis, can make autoimmune thyroid conditions such as Hashimoto's disease worse. Hashimoto's is often linked to increased gut permeability ("leaky gut") and chronic inflammation.

Gut bacteria also help convert T4, the inactive form of thyroid hormone, into T3, the active form. When gut health is poor, that conversion suffers, and tiredness and brain fog get worse. Constipation, bloating or unexplained weight change can therefore be a sign of a thyroid problem underneath.

Autoimmune conditions travel together. People with Hashimoto's or Graves' disease are several times more likely to have celiac disease, and people with celiac disease are more likely to have thyroid disease. Autoimmune gastritis, which damages the stomach lining and causes vitamin B12 deficiency, is also more common in people with autoimmune thyroid disease. Both are found with blood tests.

Gut conditions block thyroid medication. Levothyroxine, the standard treatment, is absorbed in the small intestine and needs stomach acid. Absorption falls with:

• Celiac disease
• Stomach infection with H. pylori
• Autoimmune gastritis
• Acid-reducing medications
• Calcium and iron supplements
• Coffee, fiber supplements and food taken at the same time

Someone whose thyroid levels will not settle despite a rising dose should be checked for these. The usual instruction is to take levothyroxine on an empty stomach with water, 30 to 60 minutes before breakfast, and four hours apart from calcium or iron.

Nutrients matter. The thyroid needs iodine and selenium, and iron deficiency impairs thyroid hormone production. Too much iodine is also harmful, and kelp and seaweed supplements can trigger thyroid problems.

Supporting the gut. Whole foods, omega-3 fats and probiotic foods such as yogurt, kefir and fermented vegetables support the microbiome and reduce inflammation. A gluten-free diet is necessary for people with celiac disease.

The gut and mood

The gut and brain communicate constantly through nerves, hormones and immune signals. Most of the body's serotonin is made in the gut, where it controls movement of the bowel. Irritable bowel syndrome is closely tied to anxiety and depression, and each makes the other worse. Stress alone can change bowel habits within hours.

The thyroid, the gut and the brain work as a closely connected trio. When one is out of balance, the other two often follow. The thyroid has a clear test and a clear treatment, which is why it is checked first.

Symptoms that point to the thyroid

Underactive: tiredness, feeling cold, weight gain, constipation, dry skin, thinning hair, heavy or irregular periods, low mood, slow heart rate, a hoarse voice, puffiness in the face.

Overactive: weight loss with a good appetite, feeling hot, sweating, a fast or irregular heartbeat, tremor, anxiety, frequent bowel movements, trouble sleeping, lighter periods.

See a doctor promptly for a lump in the neck, trouble swallowing, a racing or irregular heartbeat, unexplained weight loss, or blood in the stool.

The tests that sort it out

• TSH, the first and most sensitive test
• Free T4, and sometimes free T3
• Thyroid antibodies (TPO antibodies), which identify Hashimoto's
• Celiac screen (tissue transglutaminase antibody) when gut symptoms are present or thyroid levels are hard to control
• Vitamin B12, ferritin and a blood count
• Vitamin D

Tell the provider if you take biotin, which is common in hair and nail supplements. It can make thyroid results read falsely high or low. You may be asked to stop it for a few days before the test.

Treatment

An underactive thyroid is treated with daily levothyroxine, adjusted by repeat TSH tests every six to eight weeks until levels are stable. An overactive thyroid is managed by an endocrinologist. Celiac disease, B12 deficiency and bacterial overgrowth each have their own treatment.

Do not use "thyroid support" supplements in place of testing. When researchers analyzed ten of them bought online, nine contained actual thyroid hormone that was not disclosed as a drug, in amounts that could cause harm.

What helps all three

• A diet built on whole foods, with enough protein, fiber and fish. Brazil nuts, seafood and eggs supply selenium. Iodized salt supplies iodine.
• Regular exercise, which improves bowel function and mood.
• Seven hours or more of sleep.
• Managing stress, which affects both gut and thyroid symptoms.
• Taking thyroid medication correctly, as described above.
• Regular testing if you have thyroid disease or a family history of it.

At Zen IV Spa

Our thyroid panel covers TSH, free T3, free T4 and thyroid antibodies. A licensed nurse draws it at your home anywhere in New York & New Jersey, along with B12, iron, vitamin D and other tests if needed. A licensed provider reviews the results with you by video, explains what they mean for your symptoms, and tells you whether you need treatment or a referral to an endocrinologist or gastroenterologist. Our medical director oversees every service.

See Labs & Testing and Hormone Care.

Quick answers

How does the thyroid affect mood and digestion? Thyroid hormone sets the pace of the bowel and of brain function. Too little causes constipation, low mood and slow thinking. Too much causes loose stools and anxiety.

Can gut health affect the thyroid? Yes. A healthy gut microbiome supports thyroid hormone conversion and immune balance, while gut problems can make thyroid disorders worse. Gut conditions such as celiac disease also reduce absorption of thyroid medication.

What symptoms suggest the thyroid is involved? Tiredness with constipation, feeling cold, weight gain and low mood, or anxiety with weight loss, loose stools and a fast heartbeat.

Can lifestyle changes help? Yes. An anti-inflammatory diet, stress management, regular exercise and good sleep support both the thyroid and the gut. Personal medical guidance is still recommended, and thyroid medication is needed when levels are low.

Should I go gluten-free? Only if you have celiac disease, which a blood test screens for. Do the test before changing your diet, or the result can be falsely normal.

How do I take thyroid medication for best absorption? On an empty stomach with water, 30 to 60 minutes before breakfast, and four hours apart from calcium or iron.

Tired, constipated and low? Check your thyroid with a blood draw at home: zenivspa.com/labs-testing/.

All treatments are medical services. Every guest is screened, and every treatment is provided only under the clearance, prescription or standing protocol of a licensed medical provider. Compounded peptides are not FDA-approved unless stated. Nothing on this page is a diagnosis or a promise of results.