Complex sentence: Hypothyroidism can result from de Quervain's thyroiditis, which is a condition that can transiently cause hyperthyroidism and later progress to hypothyroidism; it is believed to be viral in origin.
Split into simpler sentences: Hypothyroidism can result from de Quervain's thyroiditis, which is a condition that can transiently cause hyperthyroidism and later progress to hypothyroidism. The condition is believed to be viral in origin.