Complex sentence: After women give birth, about 5% of women develop postpartum thyroiditis up to nine months afterwards; this is characterized by a period of transient hyperthyroidism followed by transient hypothyroidism; 20--40% remain permanently hypothyroid.
Split into simpler sentences: After women give birth, about 5% develop postpartum thyroiditis which can occur up to nine months afterwards. This is characterized by a short period of hyperthyroidism followed by a period of hypothyroidism; 20--40% remain permanently hypothyroid.