During pregnancy, some people may experience thyroid dysfunction when their thyroid becomes either underactive or overactive. An underactive thyroid (hypothyroidism), which can be due to: having too little iodine in your body a pre-existing immune disorder previous damage to the thyroid gland An overactive thyroid (hyperthyroidism), which is usually a result of a condition that stimulates the thyroid gland too much, such as Graves disease. Thyroid dysfunction affects 2 to 3 in every 100 pregnancies. If you have overactive or underactive thyroid dysfunction, your doctor will monitor you closely and recommend treatment specific to your situation.