Clinicians are frequently required to determine whether patients may have either hypothyroidism or hyperthyroidism at a time when they are taking medications that alter tests of thyroid function. The effects of several commonly prescribed drugs on thyroid function are therefore summarized. Phenytoin treatment of euthyroid patients results in a 30% to 40% decrease in serum T4 and free T4 levels and either normal or slightly decreased levels of serum T3 and free T3.[26] Serum TSH is in the normal range in phenytoin-treated patients. Treatment with carbamazepine and rifampin[27] also results in subnormal concentrations of serum free T4.
A number of pharmacologic agents appear to act predominantly by decreasing the rate of production of T3 from T4 in the peripheral tissues. These agents include glucocorticoids[28] or propranolol hydrochloride[29] in high doses, oral cholecystographic radiopaque agents (iopanoic acid, sodium ipodate),[30] and amiodarone.[31] All result in decreased levels of serum T3 and free T3, normal or elevated levels of serum T4 and free T4, and, usually, normal levels of serum TSH. Large doses of glucocorticoids, however, may result in suppression of serum sensitive TSH and may also depress serum iodothyronine-binding proteins, resulting in decreased levels of serum T4 and TSH. Last, dopamine, in doses that are employed to treat cardiovascular shock, generally results in decreased levels of serum sensitive TSH.16 With the latter exceptions, however, the serum sensitive TSH value serves as a generally excellent guide for the detection of a hypothyroid or hyperthyroid state in patients receiving these medications.