The thyrotropin (TSH) receptor plays a pivotal role in thyroid gland growth, function and differentiation in the mature animal but only recently has its role in the fetus and neonate been examined. Observational studies comparing the developmental regulation of TSH receptor gene expression, with thyroid morphology, and thyroid-specific gene expression in the rodent model are reviewed in the context of older literature. Together these data strongly suggest that the TSH receptor is essential for terminal thyroid maturation and growth but is not involved in early thyroid organogenesis or migration. Consistent with the aforementioned studies in rodents, babies with a loss of function mutation of the TSH receptor as well as babies born to mothers with potent TSH receptor blocking antibodies have hypothyroidism and hypoplastic, but normally-located thyroid glands. Since the TSH receptor is probably not be expressed in human fetuses before 10-12 weeks gestation when thyroid organogenesis and migration are complete, these data provide strong evidence that human chorionic gonadotropin which peaks in the first trimester of human pregnancy could not play a role in fetal thyroid development. Similarly these data imply strongly that maternal TSH receptor Abs when present in high titer are of major importance in influencing fetal thyroid function only after mid-pregnancy when, by analogy with rodents, increased TSH receptor expression is likely to occur.