L-Thyroxine is a synthetic thyroid hormone 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, alleviating symptoms such as fatigue, weight gain, dry skin, and depression. Additionally, it is used in the management of goiter, an enlargement of the thyroid gland, as a supplement in thyroid cancer therapy following thyroidectomy to prevent recurrence, and for congenital hypothyroidism in newborns to ensure proper growth and development. In some cases, it is used off-label to treat certain types of infertility and recurrent miscarriage linked to thyroid dysfunction, or for thyroid suppression tests to aid in diagnosis. Proper dosage is critical, as both under- and over-treatment can lead to adverse effects. Regular monitoring of thyroid function tests is essential to optimize therapy.