Significance of low thyroid-stimulating hormone in pregnancy.

PURPOSE OF REVIEW: To describe the significance of low thyroid-stimulating hormone in pregnancy.

RECENT FINDINGS: Interpretation of thyroid function in pregnancy must be seen in the context of changes in thyroid economy associated with gestation. Improvements in thyroid-stimulating hormone assay methodology have resulted in accurate identification of low and suppressed thyroid-stimulating hormone in pregnancy, and recent studies of screening thyroid function in pregnancy have found an incidence of low thyroid-stimulating hormone in up to 18% of women. Normative gestational-related reference ranges for thyroid hormones, particularly thyroid-stimulating hormone and thyroxine, should be established to identify the higher as well as the lower limits of these analytes. The adverse obstetric and neonatal outcomes associated with hyperthyroidism due to Graves' disease mean that this must be differentiated from the more common cause of suppressed thyroid-stimulating hormone, i.e. gestational transient thyrotoxicosis. It is suggested that estimation of thyroid peroxidase antibodies may be a useful initial diagnostic strategy in the evaluation of women with a low or suppressed thyroid-stimulating hormone.

SUMMARY: In addition to identifying women with high thyroid-stimulating hormone levels at screening (with implications for child intelligence), establishing the cause of low thyroid-stimulating hormone will improve obstetric outcome in a number of pregnant women.