Final answer:
The main symptom of iodine deficiency is the enlargement of the thyroid gland, also known as a goiter.
Step-by-step explanation:
The main symptom of iodine deficiency is the enlargement of the thyroid gland, also known as a goiter. Iodine is necessary for the production of thyroid hormones, and when there isn't enough iodine in the diet, the thyroid gland enlarges in an attempt to produce more hormones. This enlargement is a visible indication of iodine deficiency.