Hypothyroidism decreases and slows down the GI tract movement, and this often results in constipation. Other GI problems that can be caused by hypothyroidism are IBS and even intestinal obstruction, in severe cases of hypothyroidism.
Hyperthyroidism (too much thyroid hormone in your system) can cause multiple digestive system malfunctions. It can affect the functioning of the muscles of the esophagus, and can cause abdominal pain and difficulty swallowing. Too much thyroid hormone can also alter the emptying of the stomach after food consumption, and this can cause you to have nausea and even vomiting. Hyperthyroidism can also make the stomach produce less acid, and this disturbs food digestion in the stomach. Another common GI symptom of hyperthyroidism is the increased frequency in bowel movements and diarrhea. The diarrhea may be associated with an increased content of undigested fat in the stools. Diarrhea and increased frequency of bowel movements typically resolve with treatment of the overactive thyroid. If you are being treated with thyroid medication for hypothyroidism and start experienced increased frequency of bowel movements, consider thyroid hormone over-medication as a possible cause of your GI tract symtpoms. Hyperthyroidism can also cause constipation. This has to do with a significant disturbance of the contraction process of the intestines and colon.