Pregnancy-related hormonal changes and thyroid growth: do they have an impact on the higher incidence of differentiated thyroid cancer in women?

PURPOSE OF REVIEW: To analyze whether pregnancy could play a role in the higher prevalence of differentiated thyroid carcinoma (DTC) in women. Estrogens strongly modify thyroid economy by increasing iodine clearance, thyroid hormone requirement and production. Human chorionic gonadotropin (hCG) contributes to the increased thyroid hormone synthesis. Both estrogens and hCG can interfere with the regulation of thyroid volume and with thyroid nodule development and progression. The potential effect of hCG is exclusively related to its weak agonistic activity on TSH receptor. Estrogen implication on normal and nodule-derived thyrocyte growth has been demonstrated in vitro and in animal models. Furthermore, there is solid clinical evidence showing a promoting effect of pregnancy on thyroid volume and nodule development. Two metaanalyses, one including retrospective and another prospective observational studies, failed to show an association between pregnancy and DTC. RECENT FINDINGS: A large pooled prospective analysis using multivariable-adjusted Cox proportional hazard models did not demonstrate an association between DTC and parity. Similarly, no association between PTC occurrence and parity was observed in a prospective cohort analysis by linkage to the statewide Surveillance, Epidemiology, and End Results (SEER). SUMMARY: The presently available evidence does not support an involvement of pregnancy in DTC etiology.