The Effects of Phenytoin on Thyroid Function Tests: A Case Mimicking Central Hypothyroidism

Introduction: Phenytoin, a widely used antiepileptic, can alter thyroid hormone metabolism and laboratory assays, potentially mimicking central hypothyroidism. Case Presentation: A 75-year-old woman on long-term phenytoin presented with nonspecific symptoms and thyroid function tests showing low free T4 with a normal TSH. Workup revealed no evidence of pituitary disease. Elevated phenytoin levels and the absence of clinical hypothyroid features suggested phenytoin-induced assay interference or altered metabolism. Conclusion: In patients on chronic phenytoin therapy, discordant thyroid function tests may not indicate true hypothyroidism but rather drug-induced changes. Clinical correlation is essential before initiating unnecessary treatment.