Used in the treatment of hypothyroidism, where the thyroid gland does not produce enough thyroid hormone. It works by replacing or providing the missing thyroid hormone (T4), helping to regulate metabolism, energy levels, and overall growth and development. Commonly prescribed for adults and children with underactive thyroid, it alleviates symptoms such as fatigue, weight gain, dry skin, and sensitivity to cold.

It is also used in the management of goiter and certain types of thyroid nodules, particularly those due to chronic iodine deficiency.

In thyroid cancer patients post-surgery, it is used to suppress TSH (thyroid-stimulating hormone) secretion from the pituitary gland, which may stimulate cancer cell recurrence.

Additionally, it is employed in thyroid function tests, such as temporary discontinuation to measure hormone levels or in conjunction with radioactive substances for diagnostic procedures.

Taken orally, it requires consistent daily use and regular monitoring of thyroid function to adjust dosage. It should be used under medical supervision, as excessive intake may lead to thyrotoxicosis, causing symptoms like palpitations, sweating, weight loss, and osteoporosis.