Since you have been taking Thyronorm (thyroxine sodium) for 13 years and are experiencing slow growth, it is important to consult a pediatric endocrinologist. Your body's thyroid hormone requirements change as you grow, so your doctor will likely order blood tests (TSH and Free T4) to see if your current dose needs adjustment. Additionally, they may check your bone age and screen for nutrient absorption issues, like celiac disease, which can keep you underweight. Please do not change your dose without medical supervision. Your doctor will help tailor a safe, supportive plan to encourage healthy growth and weight gain.