Thyroid function during pregnancy and the postpartum period: iodine metabolism and disease states

To review various aspects of thyroid function during and early after pregnancy.

We discuss biochemical and potential pathologic changes in the thyroid associated with the gestational and postpartum periods.

Urinary iodine excretion during the last trimester of gestation in healthy euthyroid women shows that, in areas with mild iodine intake, iodine supplementation is necessary during pregnancy and the postpartum period. This measure should be considered in iodine-sufficient areas as well. Thyroglobulin is the main biochemical marker of persistent thyroidal stimulation. Alterations in thyroid volume during pregnancy can persist after delivery, especially in breast-feeding mothers. In most patients, the goitrogenic stimulus of pregnancy can be suppressed with iodine supplementation. Autoimmune thyroid disease during pregnancy and the postpartum period is reflected by monitoring of thyroid peroxidase antibodies (TPO-Ab). Women with positive test results for TPO-Ab early in gestation showed a highly significant decrease in free thyroxine and increased thyroid-stimulating hormone levels late in gestation. The main marker of Graves' disease during pregnancy is thyroid-stimulating antibodies. Nonautoimmune gestational hyperthyroidism differs from Graves' disease in that thyroid-stimulating antibodies are not detectable.

Clinicians should be alert to the fact that pregnancy can induce thyroidal pathologic conditions.