Suppression of the pituitary thyroid axis after cardiopulmonary bypass in the neonate.

Thyroid hormone has numerous effects on cardiovascular function in the adult. The present study was undertaken to evaluate the effects of cardiopulmonary bypass and deep hypothermia on thyroid function in the neonate. Ten newborns were studied preoperatively and postoperatively. The total and free triiodothyronine, total and free thyroxine, thyroid-stimulating hormone, and thyroglobulin levels were measured by immunoassays. The data demonstrated a transient rise in the free thyroxine level associated with and followed by significant reductions in the free and total triiodothyronine, total thyroxine, thyroid-stimulating hormone, and thyroglobulin levels in the early postoperative period. By the fifth postoperative day, the free and total triiodothyronine and total thyroxine levels were returning toward the preoperative levels under the influence of an elevated thyroid-stimulating hormone level. These results suggest that the combination of cardiopulmonary bypass and deep hypothermia can result in a transient suppression of the pituitary-thyroid axis in the neonate.