Amongst the multiple drug effects on thyroid physiology and diagnostic tests, numerous agents may alter the bioavailability of oral thyroxine (T4). The list includes cholestyramine, soy bean formulations, sucralfate, ferrous sulfate and aluminium hydroxide which have each been shown to diminish the normal efficient absorption of oral T4. Diphenylhydantoin, phenobarbitone, carbamazepine and rifampicin increase hepatic metabolism of T4. With all of the above agents, an increase in T4 dosage may be necessary, as best judged by measurement of serum thyroid stimulating hormone (TSH).
In hypothyroid women with breast cancer who are treated with T4, androgen in the form of fluoxymesterone has been associated with a sustained increase in free T4 and lowering of TSH. This effect is attributed to a decrease in the concentration of thyroxine binding globulin. Based on TSH levels, a reduction of T4 dosage by up to 50% may be necessary.