Cretinism is hypothyroidism in children. Failure of thyroid secretion causes retardation of growth of all forms (physical, mental and sexual) in young ones. Cretinism occurs from the deficiency of thyroid hormone.
Myxoedema:
On the other hand, myxedema is caused in adults due to the deficiency of thyroid hormones. The patient attains puffy appearance and lacks alertness, intelligence and initiative.