L-Thyroxine Sodium Salt Pentahydrate is primarily used in the treatment of hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormone. It helps to restore the normal balance of thyroid hormones in the body, which is essential for maintaining metabolism, energy levels, and overall health. Additionally, it is prescribed to prevent the recurrence of goiter (an enlarged thyroid gland) after thyroid surgery and to manage certain types of thyroid cancer. In some cases, it is also used in diagnostic tests to evaluate thyroid function. Proper dosage is critical, as both insufficient and excessive amounts can lead to adverse effects.