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Phenytoin- Medication That Warrants Deviation From Standard Approach for Thyroid Lab Interpretation
Vishwanath Pattan1, Narsimha Candula2, Ramesh Adhikari3
1Endocrinology, Wyoming Medical Center, Casper, USA.
Cureus
|December 7, 2020
Summary
Phenytoin, an antiepileptic drug, can falsely lower thyroid hormone levels, complicating diagnosis. Monitoring thyroid stimulating hormone (TSH) is crucial for patients on long-term phenytoin therapy.
Area of Science:
- Endocrinology
- Pharmacology
- Clinical Chemistry
Background:
- Long-term use of antiepileptic drugs (AEDs), such as phenytoin, is known to affect thyroid hormone levels.
- This alteration can lead to difficulties in interpreting thyroid function tests for clinicians.
- Accurate thyroid monitoring is essential for patient management, especially in those with chronic conditions.
Observation:
- A 48-year-old female on long-term phenytoin therapy presented with discordant thyroid lab results.
- She exhibited a low free thyroxine (free T4) level (0.68 ng/dL) with a normal thyroid stimulating hormone (TSH) level (1.76 µIU/mL).
- The patient was clinically euthyroid with a normal free triiodothyronine (free T3) level.
Findings:
- Phenytoin was identified as the cause of artifactually decreased free T4 levels in routine laboratory assays.
- This drug-induced effect mimics central hypothyroidism but does not reflect true thyroid dysfunction.
- Thyroid stimulating hormone (TSH) levels remained normal throughout the monitoring period.
Implications:
- Clinicians must be aware of phenytoin's interference with thyroid hormone assays.
- Relying solely on free T4 levels can lead to misdiagnosis of hypothyroidism.
- Thyroid stimulating hormone (TSH) measurements are a more reliable marker for monitoring thyroid function in patients taking phenytoin.
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