Thyroid-function tests in diphenylhydantoin-treated patients

Area of Science:

• Endocrinology
• Clinical Chemistry

Background:

• Thyroid hormone assays are crucial for diagnosing thyroid dysfunction.
• Diphenylhydantoin (phenytoin) is a commonly prescribed anticonvulsant medication.
• Potential drug interactions with thyroid function tests require careful consideration.

Purpose of the Study:

• To evaluate the impact of diphenylhydantoin on free-thyroxine index measurements.
• To determine if diphenylhydantoin interferes with commonly used thyroid hormone assays.
• To assess the validity of the free-thyroxine index in euthyroid patients on diphenylhydantoin therapy.

Main Methods:

• Comparison of free-thyroxine index in normal adults and euthyroid patients taking diphenylhydantoin.
• Serum thyroxine and triiodothyronine uptake measured using two competitive protein-binding assays.
• Investigation of potential drug interference with assay methods.

Main Results:

• Diphenylhydantoin therapy was associated with decreased mean serum thyroxine concentrations.
• Triiodothyronine uptake remained unaffected by diphenylhydantoin.
• A significant percentage of drug-treated patients exhibited subnormal free-thyroxine indexes.
• Assay methodology did not show direct drug interference with thyroxine binding.

Conclusions:

• The free-thyroxine index may be reduced in patients taking diphenylhydantoin due to lower serum thyroxine levels.
• The free-thyroxine index may not reliably estimate thyroid status or free thyroxine concentration in these patients.
• Clinical interpretation of thyroid function tests in patients on diphenylhydantoin requires caution.