[Interference in the conversion of T4 to T3 and rT3 by medications in man]

Several drugs are known to interfere in thyroid hormone metabolism: 1) They can affect directly and specifically T4 to T3 conversion. This can be observed in vivo and in vitro. In vivo serum T3 concentrations will decrease and rT3 concentrations increase. Amiodarone, other iodinated compounds (cholecystographic agents: ipodate and iopanoic acid), propylthiouracil and propranolol are known to act in this way. 2) Other drugs which induce the hepatic capacity to catabolize drugs will also affect thyroid hormone metabolism, e.g. diphenylhydantoine. Clinically, inhibitors of conversion will increase serum total and free T4, however this finding per se does not proof hyperthyroidism as in some cases serum TSH will increase after i.v. injection of 400 micrograms TRH. Furthermore the increase in serum T4 can be explained in part by a low metabolic clearance rate, T4 production being barely increased.