Hi, Thyronorm (levothyroxine sodium) is a medication commonly used to treat hypothyroidism (an underactive thyroid gland). Some of the common side effects of thyronorm include headache, nausea, vomiting, diarrhea, weight loss, nervousness, insomnia, increased appetite, sweating and heat intolerance. Black stool is not a commonly reported side effect of Thyronorm. However, it is possible that it could be a side effect, as any medication can cause unexpected side effects in some individuals. It is important to speak with your doctor if you are experiencing black stool, as this could be a sign of gastrointestinal bleeding or another medical issue. Your doctor may recommend additional testing or may adjust your medication dosage or treatment plan if necessary. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Priyanka, General & Family Physician