How long does it take for levothyroxine to start working?

Takes its time. : The full effect of levothyroxine is not immediate. When we start the medication anew or change its dose, we usually have to wait at least three weeks to perform blood tests to see its full effect. So it does take its time.

Some people will begin to feel the effects of levothyroxine after 3 weeks, but others may not feel its effects until 6 weeks. Doctors commonly schedule appointments 3-6 weeks after starting levothyroxine for the first time or changing the dose of levothyroxine to see how the medication is working and run another thyroid blood test.