The effects of cardiopulmonary bypass on thyroid function in infants weighing less than five kilograms.

Triiodothyronine is an important regulator of cellular metabolism and may have potential use as an inotropic agent. The aim of this study was to determine the effects of cardiopulmonary bypass on thyroid function in infants weighing less than 5 kg. Serial measurements of triiodothyronine, thyroxine, and thyroid-stimulating hormone were made in 10 infants and corrected for the effects of hemodilution. We demonstrated a fall in triiodothyronine and thyroxine levels, with some recovery after 3 to 6 hours. An additional decrease then occurred, reaching a trough at 48 hours (representing a fall of 78% for triiodothyronine and 57% for thyroxine) before hormone levels returned to normal at 5 to 7 days. Thyroid-stimulating hormone concentrations increased and decreased, predating and complementing exactly the changes in triiodothyronine and thyroxine. These results are quantitatively and, for thyroid-stimulating hormone, qualitatively different from those previously reported in adults. In two patients who died, however, and in one who had a particularly difficult postoperative course, no increase in triiodothyronine, thyroxine, or thyroid-stimulating hormone concentrations was found after a trough had been reached at 48 to 72 hours, which suggests abnormal function at the hypothalamopituitary level.