Since thyroid hormone occurs naturally in the body, almost anyone can take levothyroxine. You should not use this medication if you have had a heart attack, a thyroid disorder called thyrotoxicosis, or an adrenal gland problem that is not controlled by treatment.Before taking this medication, tell your doctor if you have heart disease, coronary artery disease, anemia (lack of red blood cells), diabetes, problems with your pituitary or adrenal glands, or a history of blood clots. If you use insulin or take diabetes medicine by mouth, ask your doctor if your dose needs to be changed when you start using levothyroxine.Different brands of levothyroxine may not work the same. If you get a prescription refill and your new pills look different, talk with your pharmacist or doctor.It may take several weeks before your body starts to respond to this medication. Do not stop taking this medication suddenly. Even if you feel well, you may still need to take this medicine every day for the rest of your life to replace the thyroid hormone your body cannot produce.There are many other medicines that can affect levothyroxine. Tell your doctor about all the prescription and over-the-counter medications you use. This includes vitamins, minerals, herbal products, and drugs prescribed by other doctors. Do not start using a new medication without telling your doctor.