Hypothyroidism The thyroid gland is a small butterfly-shaped structure located in the front of the neck. Hypothyroidism (low thyroid function) occurs when the thyroid gland does not produce enough of a substance called thyroid hormone, which helps to regulate the body's metabolism. Hypothyroidism develops in about 5% of the population in the United States and is especially common in women older than 60 years. Causes of hypothyroidism Hashimoto's thyroiditis is the most common cause. In this cond