Diarrhea and malabsorption are common manifestations of hyperthyroidism, whereas constipation or obstipation frequently occur in hypothyroidism. Abnormalities of gastrointestinal motility have been proposed as the primary cause of these complaints, but documentation has been conflicting and largely limited to observations of the transit time of a barium meal. We studied gastrointestinal transit time in fasting patients with thyroid dysfunction using the pulmonary excretion of H2 after the ingestion of a nonabsorbable carbohydrate, lactulose, as an indicator of the rate of transit to the colon. Mean transit time of 10 hyperthyroid patients (29 +/- 4.0 min) was significantly less than that of 42 healthy controls (72 +/- 3.7 min, p less than 0.001), and of 6 hyperthyroid patients when they became hypothyroid after treatment (80 +/- 11.0 min, p less than 0.05). Transit time decreased significantly when hypothyroid patients were given thyroid replacement (p less than 0.01). These findings support the hypothesis that abnormal gut motility may be the primary cause of the diarrhea and malabsorption of hyperthyroidism, and the constipation and obstipation commonly seen in hypothyroidism.