Thyroid illnesses are common and has a hereditary link. There are no data from India to predict risk but data from Scotland shows that the risk is about 25% if you have a first degree relative with either hypo or hyper thyroidism. Obviously if both parents have the illness the risk is higher. I don't know or can't give you exact percentage. On the other hand most thyroid illness are not serious and has good/ safe treatment options. I don't think it as a serious illness to avoid marriages