MS-1903

A 36-year-old female patient visits her GP complaining of persistent fatigue, weight gain, and intolerance to cold. Upon conducting blood tests, the doctor discovers a haemoglobin level of 135 g/L, elevated thyroid-stimulating hormone, and decreased free T4. The patient is diagnosed with hypothyroidism and prescribed levothyroxine.

What is the mode of action of levothyroxine?