Final answer:
The most common cause of thyroid enlargement in children, including those with trisomy 21, is goiter due to iodine deficiency. Graves' disease is an example of hyperthyroidism with excess thyroid hormones. Hypothyroidism is underproduction of thyroid hormones, potentially leading to thyroid enlargement.
Step-by-step explanation:
The most common cause of thyroid enlargement in children, which occurs with increased frequency in children with trisomy 21 (Down syndrome), is due to insufficient iodine in the diet. This condition is known as a goiter. Iodine is essential for the production of thyroid hormones triiodothyronine (T3) and thyroxine (T4). When there is a lack of iodine, the thyroid gland can't produce enough of these hormones, resulting in an increased production of thyroid-stimulating hormone (TSH) which causes the thyroid gland to enlarge. Supplementing iodine through diet can prevent and treat this condition in places where iodine deficiency is common. However, iodine treatment is not effective for thyroid enlargement caused by Hashimoto's thyroiditis, an autoimmune disorder.
The condition Graves' disease is an example of hyperthyroidism where too much thyroid hormone is produced. Conversely, hypothyroidism is characterized by insufficient production of thyroid hormones, often leading to an enlargement of the thyroid.