Why does hyperthyroidism cause diarrhoea?

Thyroid hormones influence metabolism and how active the digestive tract is. With an overactive thyroid, the bowel may move contents along too quickly to absorb as much water, leading to frequent, loose stools. Diarrhoea is a common presentation of Hyperthyroidism, but infection, food intolerance, medicines, and other bowel conditions can produce the same symptom.

Other features that may occur with an overactive thyroid include a fast or irregular heartbeat, heat intolerance, tremor, sweating, weight change, and feeling restless. A blood test and clinical assessment are needed to identify the cause.

Replace fluids regularly and consider an oral rehydration drink if stools are frequent. Choose small, bland meals if your stomach feels unsettled, and avoid alcohol or large amounts of caffeine, which can worsen palpitations or bowel urgency.

Arrange a medical review if diarrhoea continues or you have symptoms of thyroid overactivity. Bring a list of medicines and supplements, as some products can affect thyroid tests or bowel habits. Do not start or stop thyroid treatment without prescribing advice.

Seek urgent medical care for signs of dehydration such as very little urine, marked dizziness, confusion, or inability to keep fluids down. Get urgent help for blood in the stool, black stool, severe or worsening abdominal pain, fever with severe illness, chest pain, fainting, or a very fast heartbeat.

Diarrhoea after recent antibiotic use or travel should also be discussed with a clinician because the evaluation may differ.

Ask whether thyroid blood tests are appropriate, how to protect against dehydration, and whether any current medicines or supplements could contribute. Ask which changes in heart rate, temperature, or bowel symptoms should lead you to seek faster care.