Assessment of thyroid function during pregnancy.

Assessment of a woman's thyroid function often is necessary during pregnancy, since either uncorrected hypothyroidism or hyperthyroidism can adversely affect pregnancy outcome. Pregnancy-induced changes in thyroid hormone economy, particularly the major alterations in thyroid hormone binding to serum proteins, change the results of some commonly used thyroid hormone measurements. In vivo isotopic testing, including scintiscanning and thyroid radioiodine uptakes, cannot be used. Because of advances in assay technology, the best strategy for thyroid function assessment is now based on a high sensitivity serum thyroid-stimulating hormone (S-TSH) measurement as the first-line test. When S-TSH levels are high, indicating hypothyroidism, follow-up tests include serum free T4 and, if appropriate, antithyroid antibody measurements. The great majority of cases of hypothyroidism are due to Hashimoto's thyroiditis or prior thyroid ablation for hyperthyroidism or thyroid cancer. When S-TSH levels are low, suggesting hyperthyroidism, confirmatory tests are serum free T4 and free T3 measurements. Graves' disease is by far the most common cause of hyperthyroidism during pregnancy. Other causes, including those mediated by high hCG levels, can be distinguished from Graves' disease by careful clinical evaluation.