The influence of various anesthetics on the release and metabolism of thyroid hormones: results of two clinical studies.

Normal functioning of the thyroid gland during surgery is important, but few data are available on the possible interactions between anesthesia and thyroid hormones. In two independent studies we examined the influence of different types of general anesthesia on the plasma levels of the thyroid hormones. They revealed an intraoperative increase of free thyroxine (fT4) and total thyroxine (tT4) in plasma to approximately 150% of preanesthesia levels when enflurane was used. No increase in thyroid-stimulating hormone (TSH) secretion was noted prior to this. The increase was not due to a quantitative change in the binding proteins. The plasma levels of fT4 and tT4 returned to normal postoperatively; however, no accompanying increase in plasma triiodothyronine levels was observed. Hence we assume the increase to be due to hormone release from thyroid and/or extrathyroidal stores--an intercompartmental shifting.