Hypothyroidism is a medical condition. If a person has hypothyroidism, their thyroid gland does not make enough hormone. It can be caused by many factors, but it is more common when a person has insufficient iodine. Severe hypothyroidism in infants can result in cretinism. Symptoms of hypothyroidism include the following: tiredness, headaches, weight gain, dry skin, constipation, and poor concentration.