Abstract :
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.
Keywords :
Antiepileptic drugs, Central hypothyroidism, Drug interference, Phenytoin, thyroid function tests.References :
- Adhimoolam M, Arulmozhi R. Effect of antiepileptic drug therapy on thyroid hormones among adult epileptic patients: An analytical cross-sectional study. J Res Pharm Pract. 2016;5(3):171–4.
- Beck-Peccoz P, Rodari G, Giavoli C, Lania A. Central hypothyroidism — a neglected thyroid disorder. Nat Rev Endocrinol. 2017 Oct;13(10):588–98.
- J F, Ib L, C K, S P, Ph J, K SN, et al. The effects of phenytoin (diphenylhydantoin) on the extrathyroidal turnover of thyroxine, 3,5,3’-triiodothyronine, 3,3’,5’-triiodothyronine, and 3’,5’-diiodothyronine in man. J Clin Endocrinol Metab [Internet]. 1985 Dec [cited 2025 Jun 5];61(6). Available from: https://pubmed.ncbi.nlm.nih.gov/4055984/?utm_source=chatgpt.com
- OPPENHEIMER JH, FISHER LV, NELSON KM, JAILER JW. DEPRESSION OF THE SERUM PROTEIN-BOUND IODINE LEVEL BY DIPHENYLHYDANTOIN*. J Clin Endocrinol Metab. 1961 Mar 1;21(3):252–62.
- Surks MI, DeFesi CR. Normal serum free thyroid hormone concentrations in patients treated with phenytoin or carbamazepine. A paradox resolved. JAMA. 1996 May 15;275(19):1495–8.
- Miyake Z, Ishii K, Tamaoka A. Hypothyroidism induced by phenytoin and gabapentin: A Case Report. Medicine (Baltimore). 2018 Oct;97(43):e12938.
- Pattan V, Candula N, Adhikari R, Kashyap R. Phenytoin- Medication That Warrants Deviation From Standard Approach for Thyroid Lab Interpretation. Cureus. 12(11):e11324.
