My daughter has been diagnosed with hyperthyroidism, her dictor advised her to take a month leave from school, is this really necessary?

Leave: Unless you and your daughter think she should take time off unless she was treated with radioactive therapy. In that case she should stay home for a week. If possible she should use a toilet that no one else should use for that week.

Hard to say: We cannot assess the effect this disease has had on your daughter with the information submitted.Nor can we predict her response to treatment. If the disorder is mild and has not affected her activities/ability to study/etc, less time off might work, but the treating doc is in a better position to decide. You could always get a second opinion locally.