Hi, L-Thyroxine (also known as Levothyroxine or Synthroid) is a medication used to treat an underactive thyroid gland (hypothyroidism). Shortness of breath is not a common side effect of L-Thyroxine. However, some people may experience shortness of breath as a symptom of hypothyroidism, and taking L-Thyroxine can improve this symptom. There are other possible side effects of L-Thyroxine, which may include headache, nervousness, irritability, sweating, increased appetite, weight loss, diarrhea, insomnia, tremors and hair loss. If you are experiencing shortness of breath or any other unusual symptoms while taking L-Thyroxine, it is important to talk to your doctor. They can help determine if the medication is the cause of your symptoms or if there is another underlying condition that needs to be addressed. It is also important to let your doctor know about all the medications you are taking, as some medications can interact with L-Thyroxine and cause side effects. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Priyanka, General & Family Physician