This chapter describes the diagnosis, treatment, prognosis, and lactation for thyroid cancer in pregnancy. A few studies suggest that thyroid stimulation during both pregnancy and lactation may result in transient increase in risk of papillary thyroid cancer especially among women diagnosed with thyroid cancer at a younger age. Ultrasound may be used to characterize the nodule, detect other nodules missed on physical examination, measure nodule growth during pregnancy, and to guide the fine needle aspiration. Radioiodine therapy is contra-indicated during pregnancy as it can cross the placenta and cause fetal hypothyroidism and cretinism. The risk of malformations when chemotherapy is administered in the first trimester has been estimated to be around 7.5-17% for single agent chemotherapy and 25% for combination chemotherapy. Current information based on follow-up studies of women having subsequent pregnancies after radioiodine treatment have failed to show statistically significant effects on chromosomal abnormalities, congenital malformation, and childhood malignancies.