There are two main types of thyroid disorders:
Underactive thyroid (hypothyroidism): This condition happens when the thyroid doesn't make enough hormone. Hypothyroidism can cause weight gain, fatigue, brittle hair and nails, an intolerance for cold, constipation, irregular periods, or infertility. Lack of dietary iodine is the most common cause of hypothyroidism worldwide. In the U.S., the most common cause is Hashimotos's disease (also called chronic thyroiditis), a condition in which the immune system attacks and destroys the thyroid gland.
