Epidemiologic studies indicate that 0.4 percent of pregnant women have serum thyrotropin concentrations above 10 µU per milliliter at 15 to 18 weeks of gestation.3,4 Thus, in the United States alone, a minimum of 12,000 to 16,000 infants are born each year to women with either inadequately treated or undiscovered primary hypothyroidism.5,6 During pregnancy, women’s thyroid physiology undergoes well-defined changes, including an approximate doubling in thyroxine-binding globulin concentrations due to increases in estradiol concentrations, as well as a 30 to 40 percent increase in plasma volume.7-11 These changes result in a significant increase in the total thyroxine pool, primarily during the first trimester.