Changes in pituitary–thyroid function occur in patients with virtually all illnesses and those undergoing major surgical procedures. Although such changes are referred to as the euthyroid sick syndrome, the key changes — namely, decreases in serum triiodothyronine and thyroxine concentrations — have multiple causes, vary considerably in different patients, and very likely have different effects on different tissues. Although we have generally assumed that the decreases have no pathophysiologic consequences, it is by no means clear that the patients are in fact euthyroid.

The most common change is a decrease in extrathyroidal conversion of thyroxine to triiodothyronine, the active form . . .