I have been diagnosed with 13% of thyroid since last year, and as per doctor instruction I am taking thyronorm of 50 mg daily twince. How to prevent it?

No prevention. If your thyroid function was that low when you started thyroid hormone, it likely is a permanent form of hypothyroidism (such as hashimoto's thyroiditis) and will not improve. You likely will need to continue thyroid hormone replacement indefinitely, but don't worry, this is perfectly safe and necessary for your health.