Goiter, medical condition causing the front of the neck to swell, due to an enlargement of the thyroid gland. Goiters occur because the thyroid gland is too active (hyperthyroidism) or not active enough (hypothyroidism). When hyperthyroidism occurs, the thyroid gland produces too much thyroxine and the gland enlarges. Symptoms of this type of goiter include weight loss and nervousness. When hypothyroidism takes place, the pituitary gland secretes too much thyroid-stimulating hormone, causing swelling. Symptoms include physical and mental slowness and weight gain.