Effects of thyroid dysfunction on the metabolism of halothane, enflurane and methoxyflurane in rats.

The effect of thyroid dysfunction on the metabolism of halothane (100 ppm), enflurane (100 ppm) and methoxyflurane (300 ppm) was investigated during application by inhalation. In male rats the elimination half-lives from the atmosphere of the exposure system amounted to 0.76h for halothane, 6.84h for enflurane and 0.64h for methoxyflurane. Hyperthyroidism due to three daily injections of 0.1 mg/kg triiodothyronine i.p. significantly shortened the half-lives of all three inhalation anesthetics. Hypothyroidism due to operative removal of the thyroid gland affected the metabolism of halothane only as evidenced by a prolongation of the elimination half-life while enflurane and methoxyflurane half-lives remained unchanged. The observed differences in metabolic rates are explained by different metabolic pathways of the three compounds. They may be important for the manifestation of toxic effects.