Final answer:
Both iodine deficiency and toxicity can cause thyroid dysfunction and enlargement, known as goiter. Iodine is vital for the synthesis of thyroid hormones, and an imbalance can lead to a response that increases thyroid size to try to maintain hormone levels.
Step-by-step explanation:
The assertion that both deficiency and toxicity of iodine can cause thyroid dysfunction, including its enlargement known as a goiter, is indeed true. Iodine is essential for the production of thyroid hormones, triiodothyronine (T3) and thyroxine (T4). When iodine intake is insufficient, the thyroid cannot produce these hormones effectively, leading to increased production of thyroid-stimulating hormone (TSH) by the pituitary gland, which in turn causes the thyroid gland to enlarge in an attempt to capture more iodine and produce sufficient thyroid hormone.
Conversely, excessive iodine intake (toxicity) can also cause thyroid dysfunction in some cases, resulting in either hypo- or hyperthyroidism, and in some cases goiter, due to the thyroid's disrupted response to iodine levels. Mild forms of iodine excess are usually well tolerated because the thyroid can adapt to a wide range of iodine intakes; however, significant overdose can cause problems in maintaining normal thyroid function.