Hypothyroidism (underactive thyroid) occurs when your body doesn’t produce enough thyroid hormones or when the body is not efficiently using thyroid hormone. It affects women more frequently than men. It commonly affects people over the age of 60 years old, but it can begin at any age.
Hypothyroidism can take several forms, including:
• Hashimoto's thyroiditis: this is the most common type of hypothyroidism. This is an autoimmune response in which antibodies in the blood destroy tissues in the thyroid gland. As a result, the thyroid decreases in size and reduces its production of thyroid hormones.
• Post-therapeutic hypothyroidism: results from the treatment of hyperthyroidism with radioactive iodine or surgical removal of part or all of the thyroid gland. The treatment can leave the patient's thyroid unable to produce sufficient amounts of thyroid hormone.
• Goitrous hypothyroidism: results from an extreme shortage of iodine in the diet, producing a goitre in the neck. It is relatively rare in the Western world due to the introduction of iodised salt.