Maternal hypothyroidism is known to result in neurodevelopmental disorders in offspring, but whether subclinical hypothyroidism results in lower intelligence quotient (IQ) performance in progeny is an area of debate. Animal studies have shown that fetal thyroxine and triiodothyronine are primarily maternally derived before mid gestation. Other animal data reveal that fetal brain damage at a time that is analogous to the first trimester in humans can be linked to irreversible future brain damage. A large study conducted on an unselected population of pregnant women, both with known diagnosis of hypothyroidism and those who were screened but not diagnosed, found a four-point difference in the IQ levels of the offspring, raising the question of clinical significance. The endocrine community has accepted that subclinical hypothyroidism causes a significant decrease in IQ scores and has advocated for routine screening of pregnant women. However, obstetric authorities have cautioned that more research is needed before a causal relationship between subclinical hypothyroidism and lower IQ performance can be verified. Consequently, the American College of Obstetricians and Gynecologists has stated that routine screening and treatment of subclinical hypothyroidism cannot be recommended. We will review the basic science and clinical evidence for the neurodevelopmental effects of thyroid dysfunction in pregnancy.