Thyroid disease is common, and its effects on the gastrointestinal system are protean, affecting most hollow organs. Hashimoto disease, the most common cause of hypothyroidism, may be associated with an esophageal motility disorder presenting as dysphagia or heartburn. Dyspepsia, nausea, or vomiting may be due to delayed gastric emptying. Abdominal discomfort, flatulence, and bloating occur in those with bacterial overgrowth and improve with antibiotics. Reduced acid production may be due to autoimmune gastritis or low gastrin levels. Constipation may result from diminished motility, leading to an ileus, megacolon, or rarely pseudoobstruction. Ascites in myxedema is characterized by a high protein concentration. Graves' disease accounts for 60% to 80% of thyrotoxicosis. Hyperthyroidism is accompanied by normal gastric emptying with low acid production, partly due to an autoimmune gastritis with hypergastrinemia. Transit time from mouth to cecum is accelerated, resulting in diarrhea. Steatorrhea is due to hyperphagia and stimulation of the adrenergic system. Diarrhea in medullary carcinoma of the thyroid (MCT) may be due to elevated calcitonin, prostaglandins, or 5-hydroxyindoleacetic acid. Ileal or colonic function may be abnormal. The esophagus may be compressed by benign processes, but more often by malignancies. MRI and CT scans are the best diagnostic modalities. The gastrointestinal manifestations of thyroid disease are generally due to reduced motility in hypothyroidism, increased motility in hyperthyroidism, autoimmune gastritis, or esophageal compression by a thyroid process. Symptoms usually resolve with treatment of the thyroid disease.