To assess the most efficient means of monitoring thyroid status in an epilepsy clinic, total thyroxine (T4), free thyroxine stimulating hormone (TSH) were measured in 71 adult patients treated long-term with either phenytoin (DPH), carbamazepine (CBZ) or sodium valproate (VAL). Twenty-seven patients with one or more abnormal thyroid hormone results were further investigated by a thyrotrophin releasing hormone (TRH) test and clinical assessment. T4 was found to be normal in 85% on VAL, 40% on CBZ and 39% on DPH. FT4 was normal in more patients, namely 95% on VAL, 70% on CBZ and 65% on DPH. The TRH tests indicated that FT4 was the most efficient screening test for hypothyroidism in this epileptic population. We estimate that the use of FT4 alone as a screening test would have reduced by 60% the number of TRH tests required.