Hypothyroidism is defined as a condition in which the thyroid gland does not produce enough thyroid hormone, i.e. it is a deficiency of thyroid hormone.
In some parts of the world and even in some parts of the same country, iodine deficiency in the soil can lead to iodine deficiency in people. Iodine deficiency can lead to hypothyroidism because the thyroid gland uses iodine to make thyroid hormone.
However, there are many other causes of hypothyroidism including thyroid gland disease and conditions of either the pituitary gland or the hypothalamus.