What common medication is used as a thyroid hormone replacement?

Levothyroxine is a synthetic form of the thyroid hormone thyroxine (T4) and is widely used as a thyroid hormone replacement therapy. This medication is indicated for individuals who have hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormones. By providing the body with the necessary levels of thyroxine, levothyroxine helps to restore normal metabolic rates, regulate energy levels, and support the development and function of various organ systems.

The other medications listed serve very different purposes: metformin is primarily used for managing type 2 diabetes, simvastatin is a statin used to lower cholesterol levels, and losartan is an antihypertensive that helps manage high blood pressure. None of these options provide thyroid hormone replacement, which is why they are not suitable choices for this particular question.