Unaltered metabolism of phenytoin in thyroid disorders

The metabolism of phenytoin was studied in 9 hyperthyroid and 7 hypothyroid patients before and after treatment with either antithyroid drugs or 1-thyroxine. No difference in the half-life, volume of distribution and metabolic clearance rate was found between untreated hyper- and hypothyroid subjects. Furthermore, normalisation of thyroid function test did not change the kinetic parameters of phenytoin. The results indicate that the well-known correlation between thyroid function and antipyrine kinetics cannot be extrapolated uncritically to other drugs. It is concluded that no general rules are valid in the kinetics among different drugs in thyroid pathological conditions. Each drug must be evaluated by itself in different pathological conditions.