Final answer:
Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormone, and in some cases, replacement therapy is required to maintain hormone levels in the body.
Step-by-step explanation:
Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormone, which is important for metabolism. In cases of hypothyroidism due to certain conditions or removal of the thyroid gland, thyroid hormone replacement therapy is often necessary. The level of replacement thyroid hormone needs to be carefully monitored and adjusted as needed to ensure the proper functioning of the body.