Phenytoin- Medication That Warrants Deviation From Standard Approach for Thyroid Lab Interpretation

Long-term treatment with antiepileptic drugs like phenytoin has been reported to alter the thyroid hormone levels. It makes interpretation of thyroid labs hard for clinicians. We report a case of 48-year-old Caucasian female on phenytoin since year 1996 with stable seizure control, who was referred to endocrinology clinic in 2016 for evaluation of suspected central hypothyroidism due to discordant results in thyroid lab panel. Labs showed decreased free T4 level of 0.68ng/dL (reference range 0.89-1.76ng/dL) in the setting of normal thyroid stimulating hormone (TSH) 1.76 µIU/mL (reference range 0.46-4.68 µIU/mL). Clinically patient was euthyroid. Free T3 level was normal -3.82 pg/mL (reference range 2.77-5.27 pg/mL). Phenytoin was identified as the cause of the artifactual lowering of free T4 on routine assays. Therefore subsequent thyroid monitoring was done with TSH measurements. Continued follow-up of TSH remained normal over the subsequent follow-up of four years.