Hypothyroidism affects approximately 5% of the US population and is significantly more common in women and older adults. The most common cause is Hashimoto's thyroiditis — autoimmune destruction of thyroid tissue — followed by prior radioactive iodine therapy, thyroidectomy, and certain medications including amiodarone and lithium. Subclinical hypothyroidism (elevated TSH, normal T4) is even more prevalent, affecting 4–10% of adults. Treatment with levothyroxine is straightforward and highly effective, restoring normal thyroid hormone levels and resolving symptoms. Dose titration is guided by TSH measurement 6–8 weeks after any dose change. Levothyroxine should be taken on an empty stomach, 30–60 minutes before breakfast, as multiple substances including calcium, iron, and PPIs impair absorption.
